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?

DRM software allow you five computers each time you want to play on a new computer it will need t oconnectto iTunes store to authorize - if you junk a computer then you should disable it with iTunes...if you are lazy and forget and find you can't authorize music on a new computer then you can have the rights turned off for all five computers and then start again (I have not tried but this is supposed to be possible)

Some iTunes music (more expensive) has no restrictions and this is also 256 Kbit/s AAC compression (less compression than the standard 128 - so better quality).

Good Luck!
Thanks, that's interesting. I'll seek out that function in iTunes, to make sure I know where it is.