Flash Video is a container file format used to deliver video over the Internet using Adobe Flash Player versions 6–11. Flash Video content may also be embedded within SWF files. There are two different video file formats known as Flash Video: FLV and F4V. The audio and video data within FLV files are encoded in the same way as they are within SWF files. The latter F4V file format is based on the ISO base media file format and is supported starting with Flash Player 9 update 3. Both formats are supported in Adobe Flash Player and currently developed by Adobe Systems. FLV was originally developed by Macromedia.

Flash Video has been accepted as the default online video format by many sites. Notable users of it include YouTube, Hulu, VEVO, Yahoo! Video, metacafe, Reuters.com, and many other news providers.

The standards documentation for BBC Online states that the BBC now preferentially accepts Flash videos for submission, to the disadvantage of other formats previously in use on its sites such as RealVideo or WMV.

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 (collaboration between Adobe Systems and MainConcept) also support H.264 video and HE-AAC audio. All of these compression formats are currently restricted by patents.

Flash Video is viewable on most operating systems via the Adobe Flash Player and web browser plugin or one of several third-party programs. Apple's iOS devices do not support the Flash Player plugin and so require other delivery methods such as provided by the Adobe Flash Media Server.

Source:

http://en.wikipedia.org/wiki/Flash_Video