Can I ask ... why MKV? IE, if it's just a container (eg, AVI), what are its advantages (eg, over AVI)?
Are flames allowed if people are asking too basic questions? I'm kidding.
AVI was invented by MS in early 90s to deal with the AV realities of that time. Without going to great lengths of technobabble, let's just say that you can't push everything you want into an AVI file and still make it play properly. The format specs just doesn't allowed for many features that today's users might need. It had a problem with files over 2GB in size. The Open DML Avi was invented to overcome that. Then these guys came with moderns codecs, there are variable bitrates for both audio and video. Over-simplifying, in a file of given length, the 100.000 video frame is not right next to the 100.000 audio frame. But you need to keep the video playing in sync with audio, and at the same time deal with other things that have been put inside the container.
Which in these days means that I may want to put lossless audio in it, and 16 subtitles (if I'm part of the fansub scene), I want to add chapters, I may even want to add ordered chapters that play in sequence different versions of the movie (the equivalent of seamless branching on a DVD), I'd like to link certain files by some explicit logic, I may want to even embed other binaries inside the container - fonts, picture covers, 3 elephants, etc.
Also from a dev. point of view, I don't want to worry that the format I'm working with won't be able to deal with some crazy cool audio or video format that will be invented tomorrow. MKV allows for that, because that's the way it was put together as a container.
That's what makes MKV robust, versatile and flexible. 3 things that AVI isn't. If certain users are still living in the confined world of 350MB Xvid+mp3 encodes, they should open their eyes, the AV world is a lot bigger today. And way more complex.
After I wrote the above, I decided to add a part 2, which is less on the point and more political (hehe). Not aimed at anybody specifically... or aimed at everybody, any way you like it. There is a reason why CCCP is recommended. Because doing these 1-to-1 clarifications (OK, maybe 1-to-15, depending how many people will read this) it pains the people that
do know, to no end. Because there appears to be an unlimited number of people that
don't know, that come and ask again. However, there is only one way to become master of your own domain, and that is by learning. By reading. And not by blindly installing a thing and then throw your hands in the air in utter joy or despair depending if the "magic" of the newly installed thing worked or not.