Copyright is just what the name implies, the rights to copy the work. It is the inherent right of the creator of a creative work to control how, where and when the work is used or displayed, in print, on the web, or in any other form. These rights last until you sell them (allowing a work to be published, whether on the web or elsewhere does not automatically surrender your rights to it), or until they expire. Expiration of copyright occurs 70 years after the death of the creator. To automatically own the copyright to something it must be your work, not a copy of someone else's, it can't be a utilitarian object (you can't copyright your fridge), and it must be created (you can't copyright an unrealized idea).