STV stands for direct-to-video, made-for-video, straight-to-video, or straight-to-DVD and is simply the term used for a movie that will not be released in theaters and instead will go straight to DVD.
STV releases happen for more than a couple of reasons. A production studio may feel the movie or TV show has poor quality, a very controversial nature or a lack of public interest and will not do well if released in theaters (for films). For TV shows perhaps ratings are spectacularly low and the show gets canceled even though episodes were already produced. The studios will then release the DVD with the missing episodes and make revenue back from the sales and rentals.
Although the stigma of STV releases is that they are generally bad movies, independent filmmakers and small studios use them alot, as they dont have the resources to market a big budget full length feature for theater release. There are however, some very good STV releases such as the recent 'Futurama' and 'Family Guy' STV releases.