A Video DVD is a disc that plays on your standard set top DVD Player. It can hold about 120 minutes worth of video programming for a single layer disc and 240 minutes for a dual layer disc. (These times are bases on using typical video compression, certain video authoring programs can compress the video and fit even more onto a single discs, but this would sacrifice the video quality)
4.7 GB - Single Layer DVD
8.5 GB - Dual Layer DVD
You will need a special program to author your video into the proper DVD Video standard format. You can read more about DVD authoring here:
Data - DVD-ROM:
A DVD-ROM can also hold virtually any type of DATA you would like. A DVD can hold up to 4.7GBs on a single layer disc and 8.5 GBs on a dual layer disc. This DATA could be PDFs, MP3s, Microsoft Word Documents, Quicktime Movies, pretty much anything as long as you do not exceed the size limit.
KEEP IN MIND: Unlike your typical VIDEO DVD (which should be playable by anyone that has a set top DVD Player) the end recipient must have the program capable of opening the data on the DVD-ROM.
Simply putting your digital video files onto a DVD without authoring the disc to the proper format will result in inconsistent playback across different devices.