When you rip a DVD in Windows, the video is split up into a number of .vob files that are each about 1 gigabyte. Sometimes, its necessary to merge those files to create a single large .vob file. VobMerge is Windows freeware that does an excellent, fast job of such merger operations.
Update: Lee Schwingler points out that StreamClip also does merges and a whole lot more with MPEG files.