Some contests take a substantial part, if not all, of the copyright to your work.
This is good for the folks running the contest, bad for photographers who submit their work.
There’s a group called Pro-Imaging that is helping educate photographers about how entering these rights-grabbing contests negatively impact not only the photographers who enter the contests, but do harm to the entire photographic industry.
They’ve also developed a “Artists Bill Of Rights”, which are based on a set of “fair to photographer” criteria. Please take a look at the “Artists Bill Of Rights” on the pro-imaging website for more information.