The big game changers for StarCraft 2 over its predecessor are the
economy boosting mechanics. Each race has their own unique ways to
temporarily boost their economy, allowing a short influx of money at the
cost of energy (which recharges over time).
If you play as Protoss, your Nexus can cast "Chrono Boost" on any
structure that produces units or researches technology. This Chrono
Boost accelerates said structure's efficiency, making it crank out units
or unlock tech faster. Cast this on the Nexus to produce probes faster,
or on gateways to pump out units, or have it help you build up tech.
It's a little different for Terrans in that after you build a
Barracks, you can upgrade the Command Center to have the Orbital Command
ability that allows you to call down a "MULE" unit. These units can
harvest minerals at an accelerated rate for a short period of time
before breaking down.
Lastly, the Zerg queens can "spawn larvae" that can be cast upon
Hatcheries. In 40 seconds, that Hatchery will spawn an extra four larvae
increasing the ability for Zerg to field huge armies in a short period
of time.