File formats and what plays where ?

Would someone please explain to me all the various formats that are compatible with imovie and the whys and wherefores of finalising (why?) sharing and burning. so that it can be played on a TV or computer as well as a PC. I am so confused by the variety, and how imovie actually uses the clips in "events" as obviously the clips are sourced from elsewhere in the application. I have converted several movies that I have made and really don't know the difference between MOV, MP4, M4V, FLV etc. as they all seem to work in various modes of hardware, but I find it impossible to then import back into imovie events to be used again. By this time, because of conversion the files size is drastically reduced and so presumably is the resolution ? If I could only be directed to a good article that explains all this, I should be grateful as the books purchased or read only seem to describe the editing and key strokes and what is possible without explaining the important stuff relating to itunes and broadcasting under "share" It also might help me organise my filing system which is a complete mess !