I need to open a video file with extention .vob. how do i open it?

VOB files are video object files and are usually found on video DVD's. It is unusual to have a vob file apart from the DVD.

VLC (videolan.org/vlc/) is a free program that is able to play a number of different formats on the Mac.

Note if the vob file came from a commercial DVD it will most likely be encrypted and unplayable.