FLV files contain audio or video, and are usually ripped from video websites like YouTube. The “FL” in “FLV” stands for “Flash” because these files are in Adobe Flash Player format.
On Windows, you can play FLV files with many different video players, including the latest version of Windows Media Player, and the less-annoying Media Player Classic that comes with the freeware K-Lite Code Pack.
On the Mac, you can play FLV files with the versatile, and free, VLC program.
You can save a streaming Flash file by letting it play in your browser until it is fully downloaded. Then go into your browser’s cache folder and copy the file out to your desktop. The location of your cache folder depends on what browser and computer you are using, but you can Google-up the location pretty easily. The files in your cache folder might not have normal looking names, however you can locate the file by sorting in reverse-date order since it will be one of the newest in the cache. If the file is named something like “E661CFE7d01” as Firefox names its files, you can just rename it to something like “new video.flv” and you will be done.