The M2 Carbine was a mechanically simple modification of the M1 Carbine to allow fully automatic fire. The fire prototypes of the M1 Carbine had actually been selective-fire guns, but that requirement had been dropped by the time the Winchester design was officially adopted as the M1. It was a feature quickly requested by troops once the Carbine entered active service, though.
In 1944 the US Army acknowledged this, and introduced the M2. A total of 217,000 were manufactured at the end of WW2, and did see limited service on Okinawa - and then extensive use in the Korean War and Vietnam War.
The M1 Carbine has long generated controversy among those attempting to rigidly define its role, as it falls awkwardly between the notion of submachine gun and assault rifle, firing a cartridge that is clearly more powerful than a pistol round but equally clearly less powerful than an intermediate rifle round. Its originally intended role was as a personal defense weapon more effective than the 1911 pistol, and in service it always scored high marks for handiness and poor marks for combat effectiveness. The opinion of soldiers using the Carbine in either its M1 or M2 form was very much dependent on what role they expected it to serve.
In my opinion, its light weight and stock design make it a sub-optimal submachine gun, and its light cartridge makes it a sub-optimal assault rifle. But if you need a light and handy carbine to carry a lot and only use in emergencies, it is hard to beat for its time.