Copyright.

According to About.com this is the definition of Copyright.
"Copyright refers to laws that regulate the use of the work of a creator, such as an artist or author. This includes copying, distributing, altering and displaying creative literary and other types of work. Unless otherwise stated in the contract, the author or creator of a work retains the copyright.
For a copyright to apply to a work, it must be an original idea that is put to use. The idea alone cannot be protected by copyright. It is the physical use of that idea, such as an illustration or a written novel, that is covered under copyright law."
Basically, copyright protects others from using something that you've produced and from gaining profit or credit for it. Once your creation is under copyright, you and you alone can use it unless you state in your contract that you have given another person permission to use it.
The only thing that the copyright does not stop people from doing is using the general idea of your creation. For example, you've written a novel and in that novel one of your character's says "Oh my jellybeans that is one big fish!" and you describe that they're wearing a red hat and a yellow raincoat. Some time after you publish your novel, a film comes out with a scene that is similar to a part of your book, in the film one of the main characters is wearing a yellow hat and a red raincoat and is saying "Oh my jellyfish, that's a Godzilla of a creature there Phil!" Even though the person that wrote the film may have read your novel and ripped off the idea and made a scene from it, you cannot claim any profit that the film will have made because it is nothing like the part in your novel even if it is similar to it.
Sometimes people can get confused and mistake something that someone has made as an alteration of what you have produced. In some cases the person will have copied someones work and changed one thing to try and get away with it, but in some cases an artist or creator may have been influenced by your work and decided to do it in the style that you have worked in.
It can get complicated when something like this happens, but if you are careful and do not use parts of other peoples work then nothing bad will happen.
Now I know what you're thinking... Copyright is confusing, but it really isn't. If you're careful you won't get into any legal troubles. If you do want to use something that isn't yours then state who it is by, where you found it to show evidence that you own to rights to whatever you're using. If you cannot find any information about what you want to show, then ask someone if they who or can find out who it is by. Doing this will help the person that owns the copyright out and give them the credit that they deserve, because after all, everybody hates a copycat.

-Rebecca Hickey