The app has been Photos since the original iPhone OS release, although there has been an album named "Camera Roll" as long as I can remember (iPhone OS 2.0 was my first). See http://cdn.shopify.com/s/files/1/0163/8048/files/iOS-Evolutionjpg.jpg?1903
Update: The app can be called Photos or Photos.app or Photos on iOS. Apple seems to just call it Photos, though. The Camera roll refers to the default album within Photos. Photographs taken from your camera default to that album—it's the "roll" of images captured by your phone. If you sync photos from another device, they'll be in Photos, but not on the Camera Roll (they'll have their own album(s)). If you have other apps that interact with your photos on-device, they may read from the Camera Roll or may interact with other albums.
2nd Update: Having now updated to iOS 8, I now realize that "Camera Roll" has been re-termed "Recently Added" in Photos. This is a better description of what it is, since not all of its entries get there via camera. For example, a photo saved from Facebook's app will on iOS 7 and earlier appear in the "Camera Roll" even though it's not a photo from the camera.