DDAF has used several different video codecs (COmpressor DECompressor) over the years but now uses Adobe Flash Video and Microsoft WMV.
You can get the codecs for both of these formats along with others that are also used on the site in older galleries by installing the K-Lite Codec Pack. It is available for free from Free-Codecs.com
Free-Codecs.com offers a huge variety of codecs to play pretty much anything and everything you can find on the web today. If you are missing a codec to play a file check here for it.