I'm planning to buy the Nikon D5100. As I'll also buy a 200mm lens which format to go for ? And what does they mean and their differences [DX Format FX Format CX Format]
For Nikon DSLRs you'll want to get an F-Mount lens. Both DX and FX lenses are F-Mount. The new CX lenses are not. The D5100 has a DX format sensor, but FX lenses will still work. The D5100 doesn't have an internal focus motor, so if you want auto-focus to work you will need an AF-S lens, which has the motor in the lens itself.