RMVB is a multimedia format developed by RealNetworks, the makers of RealPlayer. Unlike the more common RM format, RMVB works with a variable bitrate as opposed to a constant bitrate, hence the title RMVB (RealMedia Variable Bitrate). The format is popularly used on torrent sites to distribute TV shows specially ones coming out of China.
A file with an RMVB extension can be played on Windows by video players such as VLC and RealPlayer and on the Linux platform players like MPlayer and totem can easily play the RMVB files. If your goal is to take an RMVB file and make it into a DVD or convert it into a VOB file, WinX DVD Author is an excellent freeware for that purpose. If you want to convert RMVB to the easily playable FLV format, you can use a software like SuperAVConvertor to do that.
RMVB as a format is not as popular as some other ones and many media players don’t support it, therefore, many people don’t recommend using this format if the intention is to mass distribute the content through a website or disc.