There are 3 main types of disc.  DVD-Video, DVD-ROM, and DVD-Audio. Each of these DVDs can be in DVD-5, 9, or 10, . The first classification is based on functions and the second classification is based on the capacity or playtime. The names of the DVDs are pretty self-explanatory. DVD-Video is for video, DVD-ROM is for pure data, DVD-Audio is for high quality audio, and DVD-R is the recordable version of all these DVDs. There is also DVD+R which is nothing more than a different kind of recordable disc. Most DVD writers or players do not care whether the recordable media is in + or - format. This used to be a problem some years ago when DVD was quite a new format and certain players would only recognise either the + R discs or the -R discs, but luckily as time has moved on so has the technology and this is no longer a problem.
The following table summarizes the different formats of DVDs.
Format Capacity Side / Layer Video Playtime Comment
DVD-5 4.7 GB Single / Single 2 hours The most basic kind of DVD
DVD-9 8.54 GB Single / Double 4 hours Need to be careful during authoring to choose the correct jumping point from Layer-0 to Layer-1.
DVD-10 9.4 GB Double / Single 4.5 hours Essentially a DVD-5 on each side. The disc needs to be manually flipped in order to play the other side. No artwork can be printed because both sides contain data.
DVD-18 17.1 GB Double / Double 8 hours Essentially a DVD-9 on each side. The disc needs to be manually flipped though and no printing can be done on these discs as both sides contain data.
In short the most popular is either a 5 or 9 as niether of these need to be manually turned over and both of these allow data on one side and full colour print on the other.