Curious about buying Ipod Nano but what mp3's can it play?

Ok sorry to do a new post if there is already a topic on this ( I know there is but they are a year or more old and probably not get any new posts. I am thinking about buying an Ipod nano, no specific reason for that particular model or brand just what I was looking at. I have about 15,000 mp3s on my pc and would like to be able to transfer some of them to the ipod. Now from my understanding when you buy a track from iTunes it can only be played on the ipod, but for all my existing mp3's that I have will I be able to load those on the ipod or do they have to be in a specific file format to play them on an ipod?