A:
iPad 1 is fairly thick and does not have a FaceTime camera. Also, was released in 2010.
iPad 2 is thinner and has a FaceTime camera. Released in 2011.
iPad 3 has the same dimensions as iPad 2, but includes a Retina display. Released in 2012.
iPad 4 is the same as iPad 3, but uses a lightning connector instead of the old iPod 30-pin connector for data and charging. Released in 2013.