Hi dr.inpoc and Welcome to BC!
1) I have no idea too.
2) Depending on the required codec.
3) If you get from known source, it shouldn't be a problem.
Lost? Now, if you could not get your video to play, or if your video plays without output (only audio). Then you are missing codecs. To identify what codec that you required. You can download Gspot
I recommend you to download K-Lite Codec Pack
. It consist of various codec and probably, you won't need to install any other codec in the future. (That's what I'm doing now.)
Hope this helps.