The Terrans are much like us, but more advanced technologically and they are far away from Earth, colonizing planets in galaxies light years from our Milky Way Galaxy. The Terran's enemies are the Zerg and some factions of the Protoss. With the alliance between the Terran Jim Raynor and his new Protoss friend Fenix, the Terran race has a better chance of survival against the vile forces of the Zerg.
The SCV is the most essential unit in your army. His job is to gather the crystal and vespene gas, in order to build your buildings. He also builds all of the buildings, which take take time to complete. To repair a mechanical unit or a building, select the SCV and right-click on the damaged unit or building.
The Marine is your basic fighter. Recruited from a bunker, he can attack both ground and air units. He is best suited for the bunker or in a group with a couple of medics to back him up.
The Medic is the one that heals all of your bio-units. By herself, she is almost useless. However, paired with attack units, she is a very usefull unit to have.
The unit with a flamethrower. He can only attack ground units. Some may find a use for this unit, but not me.
The Ghost can be a great asset to your army, provided you've built a nuclear silo added on to your Command Center. He has a cloaking device that keeps his presence concealed by all eaxcept the detector units (missile silo, Explorer, Zerg colonies, etc.). He can target a nuclear strike that will target an area that you tell him to.
The seige tank is a powerfull tank that gets even more powerfull in "seige mode", which it is stationary, but inflicts a decent amount of damage.