Buying in iTunes: restrictions?

I've purchased a few music videos in iTunes. It just occurred to me to ask: what are the restrictions on usage of these files?

For example, do these files play only when I use iTunes on this computer? If I get a new computer do I have to buy them again?

Do any of you know what are the restrictions?
Also I assume that the same applies to music files, audiobooks, movies, etc.....

What rights am I really buying?

Thanks!
Art