He hasn't appeared much. There was an awesome miniseries called DC 1 Million during Grant Morrison's run that introduces him. He appears mostly in that, All Star Superman, and maybe a couple of other comics.
Essentially, he is the original Superman, Kal El. He outlives all of his earth companions, including Lois Lane, gets depressed and leaves Earth, leaving what can be assumed his son or descendant of some sort, Superman Secondus in charge. Centuries later he comes back seemingly already God like. He looks the same but with white glowing eyes.
He decides to live in the sun for hundreds of centuries. Every new generation of Superman born travels to the Sun and Superman Prime emerges and grants them powers.
In DC 1 million, the whole story is that Superman Prime has decided to emerge permanently from the Sun, and some evil man made Sun, the villain of the whole miniseries, Solaris, is trying to plan his assasination, and the current generation of heroes are trying to prevent it.
It doesn't mean much though, as when he emerges from the sun he destroys Soloaris in one panel with the last Green Lantern Ring.
Then it shows him re-animating Lois Lane and creating a new Krypton. I think there was an Atom comic where he appeared and talked about how he could warp reality and time.
Here are some links to the final pages of DC 1 Million if you wan to see Superman Prime:
http://i68.photobucket.com/albums/i20/panthergod/Superman/DC1Million-SupermanPrime1.jpg
http://i68.photobucket.com/albums/i20/panthergod/Superman/DC1Million-SupermanPrime2.jpg
http://i68.photobucket.com/albums/i20/panthergod/Superman/DC1Million-SupermanPrime3.jpg
http://i68.photobucket.com/albums/i20/panthergod/Superman/DC1Million-SupermanPrime4.jpg
http://i68.photobucket.com/albums/i20/panthergod/Superman/DC1Million-SupermanPrime5.jpg