Copyright is an important topic for every musician that is serious about his work. It is also a dangerous topic to comment on. Firstly, it is a confusing web of legal issues that keeps many lawyers employed - and I am not a lawyer. And secondly, copyright law varies from country to country - though many countries have now standardized. So don't take this article as legal advice. Hopefully it will offer sensible advice to musicians in most countries. I advise you to do some research of your own, and consider paying someone qualified for some real legal advice.
Often the process of registering a copyright is what comes to mind when people hear the term copyright. However, songs are automatically copyrighted as soon as they are in tangible form, even recorded at home or simply written down. Copyrighted songs have to be original (not copies of another piece) and significant enough to constitute a work. Copyrighting work (getting it in tangible form) not only protects it from being copied or used by unauthorized parties but also is the first step to publishing material and ultimately being paid for it.