More video formats are coming out on the web and many of them require special software (called codecs) to run them.
Most of the codecs required to watch these movie clips are free. Some require a fee if you want to create your own clips.
Unfortunately, some people are taking advantage of this and packaging viruses and malware into codec bundles, taking money from users for software that is distributed free and giving out bad information about what is actually needed.
Downloading codec packs from torrents and various websites can have some nasty suprises when the user goes to install them.
For the bulk of the media formats out there, I recommend the VLC Media player. It is open source, has a GNU license and is free (no spyware, no ad banners, no viruses). It has most of the media formats (audio and video) built into it already and more are being added regularly. It can be obtained from the Videolan website: