Close Air Support, which the U.S. Military abbreviates as CAS, is a subset of air support (naturally). What differentiates it from other types is that it involves air support action whose targets are close to friendly forces on the ground. The U.S. Military's official definition says that it is:
Air action by fixed- and rotary-wing aircraft against hostile targets that are in close proximity to friendly forces and that require detailed integration of each air mission with the fire and movement of those forces. -DTIC
In sum, then, Close Air Support differs from interdiction or preplanned air support in that it is requested by ground forces in order to enhance their lethality or survivability, and is typically controlled by a Forward Air Controller either on the ground or in the air whose job is to ensure that the ground unit they are supporting receives the maximum benefit and minimum risk from the air support missions flown.
Close air support can be flown by fixed wing aircraft, either prop or jet driven; or by rotary wing aircraft. It can be flown by dedicated versions of these types such as the A-10 Thunderbolt II or AH-1W Super Cobra, or (in times of need) by armed lighter aircraft such as the UH-1 Iroquois or by fighter/bombers such as the F-16 Falcon. In more recent times, autonomous and remotely-piloted aircraft ("drones") such as the RQ-1 Predator have been tasked with providing air support to ground troops in need.