Also known as the CAR-15 (Colt Automatic Rifle - 15) and the Colt Commando, these short barrel rifles were the first AR15 versions to see action in the US military in Vietnam.
Generally, they had a shorter (11.5") barrel and collapsible stocks, and a different muzzle device called a moderator, although there were different models and variations.
Early versions of this rifle lacked a forward assist while many later models featured one. They were very similar to the M16A1's of the Vietnam era in other ways, such as selector switches (with some exceptions), barrel profiles, and A1 rear fixed sight, pistol grip, etc...
Other M16 Carbines were made as well, but these designs all eventually led to the development and use of the M4 Carbine.