How many types of DVD disc are there?

There is DVD-5, a single-sided and single-layer disc which holds 4.7GB of data; DVD-9, a single-sided and double-layers disc with capacity of 8.5GB; DVD-10, a double-sided and single-layer disc with capacity of 9.4GB; and DVD-18, a double-sided and double-layers disc holds 17GB. The re-writable DVD-RW, DVD+RW and DVD-RAM all had capacity below 3GB when first introduced. However, the latest capacity specification on these discs has approached 4.7GB of a DVD-ROM.