You can play your music on up to 5 computers at any given time. When you attempt to play the music you'll be asked to sign in with your MSN Music account. This will authorize the computer, and acquire the licenses so you can play your music.
To copy your music files to another computer, do one of the following:
Use a network to move your music.
E-mail the music to yourself, and open that e-mail on the other computer.
Burn the music to a CD (as data), and then download the contents of that CD on the other computer.
Copy the music to a portable device, such as a digital music player or a memory stick, and then plug it in to the other computer to unload the contents.