OpenGL ES offers a royalty-free cross-platform API for 2D- and 3D-graphics development. It is provided as part of the iOS SDK. Most devices support OpenGL ES 2.0 with the newest support version 3.0. Some very early units supported only OpenGL ES 1.1, but you are unlikely to encounter them.
Devices and features remain a moving target. Apple continues to introduce new units and make changes to iOS. As new devices are introduced, check Apple’s information pages, especially the technical specs. For iOS, make sure you read the release notes. In addition, you can look for summary pages on the Internet. One good source is Wikipedia: http://en.wikipedia.org/wiki/List_of_iOS_devices.