FLV is an extension used for Flash Video files.
Flash Video is a container file format used to deliver video over the Internet using Adobe Flash Player. Flash Video content may also be embedded within SWF files. There are two different video file formats defined by Adobe Systems and supported in Adobe Flash Player: FLV and F4V. The audio and video data within FLV files are encoded in the same way as they are within SWF files. The F4V file format is based on the ISO base media file format and is supported starting with Flash Player 9 update 3.
Users of the Flash Video format include YouTube, Google Video, Yahoo! Video, metacafe, Reuters.com, BBC Online, and many other news providers.
Though the Flash Video container format itself is published, most of the compression formats used with it are patented. Flash Video FLV files usually contain material encoded with codecs following the Sorenson Spark or VP6 video compression formats. The most recent public releases of Flash Player also support H.264 video and HE-AAC audio.