The AE-1 Program is an enhancement of the original AE-1, released in 1976 as the first Canon 'A' Series camera. The Program was released in 1981 with added features such as the wonderful "Program Mode" which allows for automatic setting of aperture and shutter speed, basically transforming an expensive SLR camera into an unpredictable point-and-shoot.
Another feature built into this camera was the replaceable focus screen. Canon originally provided numerous different types of focus screens for the Program for all your focussing needs.
The internal backlit light-metre was quite innovative in that it utilized its own measurement ability to check the brightness of the scene and compensated in its own display so as not to be too bright or dim when trying to read it.
Other benefits over the original AE-1 are primarily aesthetic or to improve ease of use. For instance, relocation of the ASA setting dial.
Aside from all that, however, the AE-1 is currently my 35mm camera of choice. I don't think I'd be comfortable using anything else now.