Basic principles of copyright law

Under the laws of the United States and most European countries, copyright is automatically attached to every novel expression of an idea, whether through text, sounds, or imagery. For example, your diary entries, personal letters, song lyrics, and drawings, even if they are only done in the most casual of circumstances.

Say, for example, you doodle a drawing of a fish on a piece of napkin. That drawing is copyrighted simultaneously with its creation and is your property. Your drawing cannot be copied, displayed, or exploited by any person other than you for the life of the copyright. In addition, no person other than you can create "derivative works" from the original drawing.

Note that copyright law does not protect any particular idea, just the expression of that idea. Your particular drawing of fish does not limit others to create their own expression of "fish", whether through drawing or other media.