So what does the “FLV” extension mean? FLV stands for “FLash Video”.

According to Wikipedia, Flash Video is a file format used to deliver video over the Internet using the Flash Player versions 6–10. Until version 9 of the player FLV was a proprietary format. However, the most recent public release of Flash Player also supports H.264 video and HE-AAC audio in the FLV file.
FLVs can also be embedded within SWF files.

You’ve probably seen the videos on YouTube – well, that’s Flash Video. Other sites that use this format include Google Video, and Yahoo! Video. Many television news operations are also using Flash Video on their websites.

Flash Video is viewable on most operating systems, via the widely available Adobe Flash Player and web browser plugin, or one of several third-party programs such as MPlayer, VLC media player (my favorite), Quicktime (meh), or any player which uses DirectShow filters (such as Media Player Classic, Windows Media Player, and Windows Media Center) when the ffdshow filter is installed.

Though the file format itself is open now, the codecs still used with it are patented.