There are 3 common types of DVDs: DVD-5, DVD-9, and DVD-10.
A DVD-5 is a single layer DVD that holds up to 4.7GB of data (around 120-133 minutes of video depending on compression).
DVD-9 is a dual layer single sided DVD that holds up to 8.5GB of data (around 240 minutes of video depending on compression). This DVD is comprised of a solid layer and a semi-transparent layer that allows for almost twice the data of a DVD-5 and is often used to retain a higher bitrate (quality) that may not be feasible on a DVD-5.
DVD-10 is a 2-sided DVD that holds up to 9.4GBs of data (around 266 minutes of video depending on compression). Please note that the two-sided DVD-10 is somewhat rare, and there is no interactivity between the sides. To view the information on the other side, the DVD must be removed from the player and turned over. Most times this is used when putting an NTSC and PAL video on the same disc. .
NOTE: Since the DVD-9 and DVD-10 formats are not terribly popular, to choose these when configuring your order you need to click the 'Show All' button on the 'DVD Type' page. Also, DVD-9 and DVD-10 are ONLY available on quantities >300.