There are 3 main types of video aspect ratio and video formats that exist for DVDs :
1.33 (4:3) - pan & scan : The footage is either in the original 4:3 (width:height) radio or the widescreen movie has been "cut" to fit into this ratio, meaning the footage "pans" left and right to fit in all the information. This is rare for DVD-Movies.
1.85 (16:9) & 2.35:1 - widescreen : This is more common. This is where the original theatrical aspect ratio is preserved. But since most people don't have wide-screen TVs, the top and bottom are convered with black bars. Around 20% of the picture space is wasted in these "black-bars".
1.85 / 2.35 - anamorphically enhanced : The most preferred format for most enthusiasts. This sometimes is also called 16x9 enhanced (because of the 16:9 (1.85) aspect ratio of most new movies). Basically, in an attempt to make sure the "black-bars" space isn't wasted, the original 1.85 widescreen footage is "stretched" (by going through several digital-enhancing procedures) to the full size of the screen (ie. 4:3 aspect ratio). 2.35 movies are also streched, but a smaller "black-bars" area still remains. Click here for more a detailed explanation (with diagrams).
DVD-Video extra features ...
DVD-Video also allows interactive menus, games and production material to be distributed on the same disc as the movie. Sometimes, 2 different version of the same movie can be stored on a double-sided disc, as is with the case of A Bug's Life. The sort of extra-features that have so far appeared on DVD-Video discs include hour-long making-of documentaries, production notes, cast and crew bios, theatrical trailers, pre-production sketches, special-effect explanations, audio/video commentary by the director/producer/cast, additional scenes and screen-tests. Sometimes there are also DVD-ROM material, that you can use on your computer if you have a DVD-ROM drive : things like games, puzzles, original script, screen-savers, and much much more. Some of the technologies not used yet including multiple angles and video branching. Most DVD-Video disc are more than just movies, but a full entertainment package!