WMV stands for Windows Media Video. WMV files can be played on a number of different players, but Windows Media Player was specifically built for them. It is a compressed file format, so the resolution is not high, but the file sizes are very small.
FLV stands for Flash Video Format. Videos in this format are typically played on the Adobe Flash Player. Most big video websites on the internet use this format for playback, including YouTube. Streaming video like this, rather than creating video files, is the most common way that FLV files are used.
MPEG-4 is a common format, not least because it is one of the recording options available in modern camcorders. It is extensively used when files are transferred over the Internet because the sizes are small. However, it still delivers good quality videos.
RealVideo is a format that delivers small file sizes, but it is not used today as much as it used to be. Videos in this format are compressed and must be played back on a specially-created player.