Copyright is very tricky since these days almost anything you post has to have come from someone before (meaning there is a owner). A quick search of the word copyright on the dictionary site gives you this definition.
the exclusive right to make copies, license, and otherwise exploit a literary, musical, or artistic work, whether printed, audio, video, etc.
It is a very tricky definition as you have to ask yourself why are they enforced and how does one obtain these exclusive rights in the first place? With this assignment I used the article Andy Baio is Kind of Screwed which attempts to explain something called fair use. No one really has a clear concept of copyright which is why some unlucky few like Andy gets slapped with a huge fine.
The problem with copyright is in the definition as it’s really hard to determine when your infringing on copyright. You could be doing something for fun like Andy did and not know you are infringing on copyright.