Right, converting video requires the codec that can read the source as well as a codec for whatever format that one wants to translate the video into. And, of course, some software to do the converting. Sometimes the codec is built into a player, but in many cases it is a separate piece of code. There are many common formats used, but there are also some strange non-typical ones (for example, a user may have a codec for his video camera that was installed with the camera software and such videos may not play on other systems).

And it seems worth mentioning, in some cases I had seen video posted in newsgroups that required very strange codecs be installed, and I decided against installing things that I didn't have and only needed for a particular video. In some cases this was just some encoder wanting to play with different and unusual technology, but in other I found that telling people to install a new codec was an attempt to sneak malware into their system. So even with Linux I would be vigilant about accepting a codec from strangers this way.