How Flash Video Works

Flash is a very serious force in the world of streaming video. This is the format we recommend most strongly.
Flash uses two main formats: .swf for standard Flash files which are used in web pages, and .flv which is a special Flash video format. flv files can be called from within swf files. As of late 2008 Flash also supports H.264 files, which is a significant leap forward.
http://www.mediacollege.com/adobe/flash/video/intro.html