FLV (Flash Video) is a proprietary file format used to deliver video over the Internet using Adobe Flash Player (also called Macromedia Flash Player) version 6, 7, 8, or 9. FLV content may also be embedded within SWF files. Notable users of the FLV format include Google Video, Reuters.com, YouTube and MySpace. Flash Video is viewable on most operating systems, via the widely available Macromedia Flash Player and web browser plugin, or one of several third-party programs such as Media Player Classic (with the ffdshow codecs installed), MPlayer, or VLC media player.
SWF is a proprietary vector graphics file format produced by the Flash software from Adobe (formerly Macromedia). Intended to be small enough for publication on the web, SWF files can contain animations or applets of varying degrees of interactivity and function. SWF is also sometimes used for creating animated display graphics and menus for DVD movies, and television commercials. The Flash program produces SWF files as a compressed and uneditable final product, whereas it uses the .fla format for its editable working files.
(The definitions above are quoted from wikipedia.com)