There are 9 kinds of DVDs:

DVD-1: Single sided, single layered, 8cm.
DVD-2: Single sided, dual layered, 8cm.
DVD-3: Double sided, single layered, 8cm.
DVD-4: Double sided, dual layered, 8cm.
DVD-5: Single sided, single layered, 12cm.
DVD-9: Single sided, dual layered, 12cm.
DVD-10: Double sided, single layered, 12cm.
DVD-14: Double sided, single layered on one side, dual layered on the other, 12cm.
DVD-18: Double sided, dual layered, 12cm.

DVD-5 and 9 are the most used types.