Reinstalling Leopard

Hi all, Mac noob here, sorry for a dump question but I'm very frustrated....

I have installed Leopard successfully, how can i reinstall it?
I noticed that they sent me a email with a serial number
"Dear APPLE_CUSTOMER Thanks for registering your new Mac..."

Can I reinstall Leopard on it? Or its already been licensed? Can I install this Leopard (same disk) to my other mac??

Thanks a billion mac friends