iTunes can use and convert most music formats (ie - MP3 & audio CD-ROM) into the format used on Apple iPods. With the CDs, itunes usually asked if you want to add them to the library - and you can download the 'cover art' too (altho' it's not always accurate...)
If you're not keen on submitting yet another credit card online (I'm not) you can pick up an iTunes gift card and use that to start an account on the iTunes store.
If you're looking to record something online - like that YouTube video with that darned Japanese song you can't get out of you head - and you have no idea who did it, or WORSE
it's not available ANYWHERE
and you've tried EVERYTHING
... there's WireTap Pro. Does a descent job converting sounds on the Mac into a file that iTunes can use.