The APC is a fully enclosed, wheeled vehicle that can hold up to a dozen passengers plus a driver.
During the Guild Wars the Confederate Armed Forces equipped their APCs with reactive armor. The Kel-Morian Combine pieced them together from various materials, sometimes the same reactive armor, other times they welded metal plates to their flanks, angling them to deflect bullets. These APCs were frequently armed with double-barreled gauss cannons.
Concept art exists for a unit called the "armored transport" in Wings of Liberty. The armored transport was apparently under consideration for inclusion as a regular unit, specifically one high up the terran tech tree. While it had a weak attack, it had a high armor rating. It was only capable of transporting infantry. In its final release, the APC's visual design was inspired by the shuttle from District 9.
The "militarized colonist transport" is an unused variant of the APC located in the game files. It is camouflaged and mounts a larger turret with twin chemical/napalm sprayers. It was to appear in the Char missions of Heart of the Swarm, where it would use its weaponry to melt the remaining zerg.