StarCraft II: Wings of Liberty/Getting Started

Unless you've played the original StarCraft, it's recommended that you play that game first to get to know the histories of the three factions.

In StarCraft II, you can select up to 24 units at a time.

Terran
Many Terran buildings have the ability to "Lift Off" and fly. Players can use this to move one base to another are after resources in an area have been depleted. Additionally, it can be used to save buildings from enemies who can only attack ground. While a building is in flight, it can neither recruit units or research upgrades or technology.

Protoss
The Protoss now have the ability to warp in units anywhere that energy eminating from an owned pylon exists.

Zerg
The Zerg have several distinct abilities, including the ability to morph and burrow.