The iPad doesn't have a DVD disc drive. In order to watch DVD movies on iPad, you need to convert DVD to the video format supported by iPad first. After you have the files in a compatible format transfer them to your iPad with the sync of iTunes.
Apart from those videos, movies and TV shows purchased from iTunes, common MPEG-4 video files can also be transferred to iPad for playback. For the non-MPEG-4 video files, you have to convert them first.
You are fond of movies, music videos, TV shows on YouTube but having not enough time to enjoy them all right now? It sounds a good idea to download them first, transfer them to your iPad and then take them on the go.