Copyright is the right of creators to control how their creations are used. It applies to all types of media: print, sound, images, motion pictures, etc.
Creations are automatically given copyright protection. However, in order for something to be protected it must be "fixed," which means the creator has to record it in some way -- as a print document, sound or video recording, photograph or painting, web page or e-mail. Ideas cannot be copyrighted. They need to be expressed in a tangible format.
Copyright should not be confused with trademarks or plagiarism, which are related, but different concepts. Copyright is international, with most nations having signed at least one of the many copyright treaties, the oldest of which is the Berne Convention. Although copyright is international, you should follow the law of the country in which you are using copyrighted material or publishing the creation.
Creators may also transfer their rights to others, or may release a work to be used freely by others under one of the Creative Commons licences.