The Terran units consist of human infantry and mechanized armor and air power. If you've ever played WarCraft or WC2, you can play Terrans with little difficulty. Unfortunately, Terrans are generally considered the weakest of the three sides, even if only by a slight margin. Consisting of organics mixed with technology, the Protoss warp in units and buildings through trans-dimensional gateways. Enhanced psionic powers grant them protective, regenerating shields and strange, terrible effects akin to magic.
Protoss units are costly in terms of resources, but tough as nails.
Buildings have to be within the range of energy transmitting Pylons, lest they be rendered useless. The Zerg have evolved a purely organic society. The drones that collect crystals and gas can also mutate into living buildings. Units improve by evolving the species to gain range, speed, or powerful new abilities. The lowest of Zerg units are especially cheap to breed, but have little capacity for damage.