According to that wiki entryActually he was right, yes, MKV is a wrapper format, similar to OGM. The video file in the MKV file is probably an .avi file, and using something like VirtualDubMod you would be able to extract the video/audio and save it as an .avi, without the need to convert.
http://en.wikipedia.org/wiki/Matroska
Matroska is similar in conception to other containers like AVI, MP4 or ASF, but is completely open source.
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It usually carries H.264 video, AC3/AAC/DTS (also multiple) audio tracks and sometimes subtitle tracks.