The Nether is a different dimension existing in Minecraft worlds, first existing in Minecraft Alpha version 1.2.0 on October 30th, 2010 (The Minecraft Halloween Update). 
Accessible via Portal, it can be used for travelling large distances on the above ground map quickly. The Nether also contains Block types not seen anywhere else, as well as supplies of Block types found on the normal world, like Mushrooms, Gravel and Lava. The Nether is home to four Mobs: Ghasts, Magma Cubes, Blazes, and Zombie Pigmen. All of these mobs are only found in the nether with the exception of Zombie Pigmen which spawn in the overworld when a pig is struck by lightning. 
The Nether functions as a second map in a player's Minecraft World. When it is entered, the chunks from the above ground map are unloaded and the Nether chunks begin to load. Terrain generates infinitely here just like on the above ground map. If the player dies in the Nether, their items will stay there while the player respawns in the overworld. 
Terrain: The Nether's terrain is largely composed of netherrack, which is arranged into large, smooth outcroppings and niches; it is essentially a cave. There are patches of gravel and soul sand, as well as stalactites of glowstone hanging from the ceiling. Mushrooms grow abundantly in the Nether, and natural fires are a common sight. Small tunnels form in the netherrack, which contain no ores or hidden caverns. Nether fortresses can be found as well. These Nether fortresses are made up of nether brick, Nether Brick Stairs, nether brick fences and crops of nether wart. At the very bottom, beneath an ocean of lava, is a layer of Netherrack and bedrock. The very top of the Nether is similarly bedrock. 
Navigating the Nether can be very dangerous. The strange, dimly lit landscape conceals pits and sudden cliffs. Many lava streams flow from the ceilings, down the netherrack and into a lava ocean. If one dies near this lava or a patch of fire, it is a safe assumption that some of one's items will fall in and be burned. 
The sky and fog in the Nether are dull dark red. Thunderstorms can cause the shade to flicker quickly, a known bug. 
Environment: The Nether has no day/night cycle and no weather. The only natural sources of light are fire, lava, and glowstone. A dim ambient light can be seen throughout the Nether, but this is purely a visual effect and does not affect game mechanics. Contrary to some common myths, light works the same way in the Nether as it does in the Overworld: there are 16 levels of brightness and one level falls off for every block it radiates through. 
Plants with minimum light requirements, such as flowers and Netherwart, can be planted almost anywhere in the Nether, even in complete darkness, because they can technically "see the sky" from any location. The exception, due to a bug, is anywhere below natural veins of glowstone or below lava source blocks that appear naturally in cave walls. Plants at any level below these formations, even if seperated by other terrain, will uproot if not adequately lit. 
Blocks and Structures: Entries marked with a D require additional data to fully define the block in a Beta world. Entries marked with a T have tile entities associated with them to store additional data. Items with IDs in red cannot be legitimately obtained in the player's inventory in the game; they can only be obtained by "hacking" (such as the use of Inventory editors) or, in multi player using the /give server command. Items available only in Creative mode are in blue.  
Natural Blocks: Naturally Generated includes blocks that are created through the world seed. 

Icon  Dec  Hex  Block type  
 00  00  Air  
  07  07  Bedrock  
  11  0B  Lava (stationary) D  
  13  0D  Gravel  
  39  27  Brown Mushroom  
  40  28  Red Mushroom  
  115  73  Nether Wart D  
  51  33  Fire D  
  87  57  Netherrack  
  88  58  Soul Sand  
  89  59  Glowstone Block  
 Naturally CreatedNaturally Created means a combination of events that cause a new block to be placed by natural causes, not the player. 

Icon  Dec  Hex  Block type  
 00  00  Air  
  10  0A  Lava (flowing) D  
  39  27  Brown Mushroom  
  40  28  Red Mushroom  
  49  31  Obsidian  
  90  5A  Portal  
 Structures: Same as Naturally Generated, but these blocks will only be created with the "Generate Structures" option enabled. However, due to a bug, Nether Fortresses still generate even with "Generate Structures" set to "off". 

Icon  Dec  Hex  Block type  
 00  00  Air  
  52  34  Monster Spawner T  
  112  70  Nether Brick  
  113  71  Nether Brick Fence  
  114  72  Nether Brick StairsD  
  115  73  Nether Wart D  

Nether Affected Materials:
When Grass blocks are obtained using a tool with the "Silk Touch" enchantment, such as a shovel, and taken into the Nether, it becomes possible to have grass in the nether. Using Water Buckets will only cause the water to sizzle and leave steam. This makes growing Sugar Cane impossible since the plant requires water to survive. Wheat farms are also affected by this. They can survive without water, although they will grow much more slowly. 
Actual blocks of water can be placed in the Nether, but this requires Inventory Hacking or Map Editing. Blocks of Ice that have been hacked into the game, spawned via Creative Mode, or acquired through mods can be placed and broken to leave water blocks behind as well. 
Trees will grow normally, and their leaves will take a dead-looking brownish color, like in Desert biomes. However, they grow in odd formations and occasionally morph shapes when they are altered. This makes Nether-trees useful for traps or endless log harvesting on occasion. Trees are also hard to keep in the Nether because of the substantial amount of lava, which ignites them. 
It is always a good idea to bring paper down into the Nether, and then craft a map. Getting lost is easy and happens frequently. Compasses and Clocks can be brought in freely, but they will not work correctly. As the Nether is in a totally different dimension from the Overworld, Compasses will be unable to find the original spawn point, and Clocks cannot determine the position of the sun. Their arrows and dials will instead spin and flail madly. Maps made on the above ground dimension wont show the player's position in the other dimension. For Maps to display the Nether, they must be Crafted there. Beds will explode when a player attempts to sleep in one. This can be a useful strategy to quickly mine materials in the Nether, but only if the difficulty is set to peaceful.[1] 
Lava flows twice as far in the Nether than in the Overworld (generally 8 blocks, instead of 4 blocks). 
Mobs: The Nether is home to its own share of mobs. 
Ghasts, huge jellyfish-like creatures, will lazily hover around. They are 444 blocks large, and have 9 tentacles hanging from them. If they spot the player, they will spit flaming bombs at them, which can be deflected by shooting arrows at them, punching them or hitting them with any tool. The reflected fireballs will instantly kill the Ghast, if it is hit. These will explode and often leave craters, if the blast-site is weak enough. They make screeching and whimpering noises, something like a kitten or an angry baby, and can be heard for great distances. 
Zombie Pigmen spawn in groups and wander aimlessly. They will not harm the player unless attacked. If they are, all the Pigmen within a 32 block radius rush the player, often causing ambushes from behind as the player attempts to attack while fleeing. 
Blazes are able to float and shoot 3 fireballs at the player in a quick succession. They appear to catch fire just before shooting said fireballs, then "cool down" until they can shoot again. 
Magma Cubes are spring-like mobs that slowly hop towards the player. If killed, they split into 2-3 smaller versions of the original one. Magma Cubes are occasinally rare. 
Nether mobs are completely fire-proof and can often be seen wandering into lava, where it will take them a while to get out due to the slowdown it causes on movement. 
Normal World Mobs like Pigs and Zombies will not spawn in the Nether. Chickens can be forced to spawn there if they hatch from a thrown Egg, however, but they will not spawn naturally afterward. Squid will spawn in large amounts in any hacked body of Water due to the fact that the water is the only suitable place in the whole map. Creative mode mob spawning eggs still work in the nether, so if you reach the nether in creative mode, you may have every mob. 
Portals and Fast Travel: Nether Portals are made by creating a 4x5 (2x3 on inside) door frame out of Obsidian and then lighting the inside of the frame on fire with Flint and Steel or a Ghast's fireball. When you stand inside the portal for the first time, it will create a portal in its equivalent area of The Nether. A loading screen will appear during the change of worlds. 
If you create a portal in the Nether and re-enter the normal world, any distance covered in The Nether is multiplied by 8, effectively making The Nether a fast travel zone that allows for greater distances to be covered in a short amount of time in the normal world. Linking a network of portals between the Nether and the Overworld can be tricky though. See the Nether Portal page for the precise details of how this can be done. 
Due to Ghasts, it is recommended that players enclose their portals with a blast-resistant material (Cobblestone is cheap and effective). This will prevent Ghasts from being able to hit the portal and shut it down. Optionally, the player can choose to carry a Flint and Steel with them in case this happens, or just trick a Ghast into shooting the portal again, therefore effectively re-activating the portal. 
It is not possible to get to the Nether via The End. 
History: The Nether was one of the key features to be added in the Minecraft Halloween Update, and was announced on October 4th, 2010. This announcement stated that there would be a "hell world" that could be used for fast travel. It hinted it would utilize Portals and that distance travelled inside the would be multiplied greatly on the map above ground. On October 10, a screenshot was released by Notch depicting what the Nether would look like. With the rest of the Halloween Update's features, the Nether was released on October 30. 
In the original announcement, Notch mentioned "a brand new Hell world." When the preview was sent out on 29 October 2010 to a couple of gaming websites, the new dimension was called the Slip to avoid offending religions and to have a more original name. It was referred to as 'The Nexus' on his personal blog. It was finally changed to the Nether on the day of the Halloween update release. 
Along with the Nether, Ghasts, Zombie Pigmen, Netherrack, Soul Sand, Glowstone and Portals were added. On November 10th, Lava was made to flow farther than it did on the Overworld. 
There was a glitch where The Nether was a Biome. This was fixed in an unknown update. 
In Beta 1.6, Portals were made to function properly in multi player, allowing players in servers to warp between the Overworld and the Nether.[2] 1.6 also fixed a bug where the player could have menus open while inside Portals. When warping, the text on screen would appear totally grey. 
In Beta 1.6.2 Beds in the Nether were made to explode if used. This can be used as a mining technique. 
With the "Silk Touch" enchantment in the 1.9 pre-release 4, it was possible to legitimately obtain grass and water using ice in the Nether. Because passive mobs and squids will spawn in the only place available, it was possible to use this to control the spawning, which allowed the player to make highly efficient passive mob grinder, something extremely difficult to do in the overworld since animals stopped despawning in 1.8. The ability to pick up ice blocks with the enchantment was removed in a later version . Water also made Nether mob grinders easier as it can transport items without burning them as well as drown Nether mobs that are immune to lava, such as magma cubes and zombie pigmen. 
In Minecraft 1.0.0, many new things were added to the Nether: 
Magma Cubes 
Blazes and Blaze spawners 
Nether Fortresses and the blocks they are made of 
Nether Wart 
Ghast Tears dropped by Ghasts 
File Save Location: The Nether is saved in the same way normal worlds are, but instead of mixing the world files inside the save folder, the files for Nether are stored in %appdata%/.minecraft/saves/Worldname/DIM-1 (on Windows), whereas normal world files in %appdata%/.minecraft/saves/Worldname. Note that DIM-1 does not contain its own level.dat, as the same level.dat in the upper folder is used for both Overworld and Nether. This means that even if a player changes the level data in the /world folder for a particular save, Nether portals will still take the player to the Nether world originally generated for that save. 