There's more than one kind of DVD. DVDs can be one-sided (DVD-5 and DVD-9 are one-sided) or two-sided (DVD-10, DVD-14, DVD-18 are two-sided). The various DVD designations, or "flavors of sidedness" come from the fact that one side of a DVD can have up to two layers, on which the laser pickup can focus to read data. DVD-5 is single-side, single-layer (SS SL). DVD-9 is single-side, dual-layer (SS DL). DVD-10 is single-side, single-layer (SS SL).
(The low-numbered DVD designations are seldom used—they're included on the tables below to make the columns look nicely filled up.)

For reference, a CD-ROM holds about 650 megabytes, which is 0.64 gigabytes or 0.68 billion bytes. In the list below, GB means gigabytes (2^30), GiB means billions of bytes or "gibibytes" (10^9). DVD5s hold about 5 GB of data, DVD9s hold about 9 GB, and DVD10 about 10GB.

SS = single-sided, DS = double-sided, SL = single-layer, DL = dual-layer

Designation		Sides	Layers (total)	Diameter	Capacity		Times
				(cm)	(GB)	(GiB)	(min)
DVD-1	SS SL	1	1	8	1.46	1.36	36
DVD-2	SS DL	1	2	8	2.66	2.47	67
DVD-3	DS SL	2	2	8	2.92	2.72	74
DVD-4	DS DL	2	4	8	5.32	4.95	135
DVD-5	SS SL	1	1	12	4.70	4.38	120
DVD-9	SS DL	1	2	12	8.54	7.95	215
DVD-10	DS SL	2	2	12	9.40	8.75	240
DVD-14	DS DL/SL	2	3	12	13.24	12.33	335
DVD-18	DS DL	2	4	12	17.08	15.90	435

Source: DVD on Wikipdeia, excepting play times, which are estimated and can vary with compression level and source material.
Table content is licensed under a Creative Commons Attribution 3.0 License

Designation		Sides	Layers (total)	Diameter	Capacity		Times
				(cm)	(GB)	(GiB)	(min)
DVD-R	SS SL (1.0)	1	1	12	3.95	3.68	100
DVD-R	SS SL (2.0)	1	1	12	4.70	4.37	120
DVD-RW	SS SL	1	1	12	4.70	4.37	120
DVD+R	SS SL	1	1	12	4.70	4.37	120
DVD+RW	SS SL	1	1	12	4.70	4.37	120
DVD-R	DS DL	2	2	12	9.40	8.75	240
DVD-RW	DS DL	2	2	12	9.40	8.75	240
DVD+R	DS DL	2	2	12	9.40	8.75	240
DVD+RW	DS DL	2	2	12	9.40	8.75	240
DVD-RAM	SS SL	1	1	8	1.46	1.36*	36
DVD-RAM	DS DL	2	2	8	2.65	2.47*	67
DVD-RAM	SS SL (1.0)	1	1	12	2.58	2.40	65
DVD-RAM	SS SL (2.0)	1	1	12	4.70	4.37	120
DVD-RAM	DS DL (1.0)	2	2	12	5.16	4.80	131
DVD-RAM	DS DL (2.0)	2	2	12	9.40	8.75*	240

Source: DVD on Wikipdeia, excepting play times, which are estimated and can vary with compression level and source material.
Table content is licensed under a Creative Commons Attribution 3.0 License

DVDs (or CDs) can work with either 3 inch/80mm packaging (we call this "mini dsic size") or regular "cd" size 5 inch/120mm packaging. They can also work with custom size or a taller "dvd height". The discs can be held with foam hubs, pockets, or trays. Booklets can be a more standardized size, or also made especially small in the 3 inch/ 80mm size.

 

Clear substrate discs are made by using the mini disc mold (so the same data size as a mini disc!) and clear substrate rather than the metalized substrate out to the edge of the disc. The result is a partially see through disc. The physical size is the same as a normal, full size disc, and as a result, it never gets stuck in a slot player! You can have a lot of fun with the art.