More accurately, Superman now belongs less to DC. Superman co-creator Jerry Siegel's heirs successfully won back their half of the copyright on Action Comics #1, the very first Superman comic book ever.
The reason this is even possible is due to (a) complicated extensions and rules of copyright law and (b) the fact that Jerry Siegel and Joe Shuster created the Action Comics #1 Superman story months before they sold it to DC, so it was an independent work, not work-for hire. After that issue, they became employees of DC, so all the rest of the stuff they did from then on is work for hire.
This means that DC still owns everything Superman-related that anyone thought of following that issue (inlcuding Lex Luthor and Superman being able to fly or have X-ray/heat vision), but don't fully own the other major things that make Superman Superman. For instance:
The Siegels own half of the copyright to everything introduced in the first issue of Action Comics #1, which, really, is the vast majority of everything folks associate with Superman. The name Superman, the secret identity of Clark Kent, the fact that he is an alien who came to Earth from Krypton as a baby, his super strength, invulnerability and ability to leap over tall buildings in a single bound, the red, yellow and blue costume with a red “S” on his chest and a red cape on his back, and his fellow reporter, Lois Lane. The Siegels co-own all of that, which is basically everything.
From the excellent FAQ at Comics Should Be Good!
Other interesting tidbits:
- DC still owns TRADEMARK rights to Superman, which means no one else can really advertise or market anything Superman-related, even if they lawfully create new works.
- DC will have to shell out a hunk of money to the Siegel heirs, based on whatever their fair share of Superman profits since 1999 (when they regained their half of the copyright) amounts to.
- In 2013, again due to complexities of copyright law, the rights of "heirs" vs. "estates," and the intricacies of various extensions on copyrights, Joe Shuster's estate will gain the ability to take back the OTHER half of the Action Comics #1 copyright, leaving DC with no share of that original copyright at all. DC will be left with rights to the derivative works created after the first issue (which may be useless to them, since they're derivative), plus the trademarks. So, at that point, they'll basically just be licensing Superman from the two estates, much like their parent company, Warner, now licenses the right to make Harry Potter movies from J.K. Rowling.