Copyright is one of the three branches of Intellectual Property and is a Constitutional Right enumerated under the Copyright Act. Essentially, it protects the expression of ideas, but not the ideas themselves. What's the difference? The idea of an opera about Noah's Flood cannot be copyrighted, but Benjamin Britten's expression of that idea in Noye's Fludde can be copyrighted, with the expression being the libretto and the music.
The difference between copyright and trademarks and patents is what they protect. Copyright is the most expansive of the three types of Intellectual Property (or I.P.) since it covers the expressions of ideas in various formats. According to the trademark office, trademarks are "generally a word, phrase, symbol, or design, or a combination thereof, that identifies and distinguishes the source of the goods of one party from those of others." Patents are more complicated, but they protect inventions and processes. This library guide will only discuss copyright as it pertains to music, but see the embedded links for more information about the other types of intellectual property.
Works that can be copyrighted must be original works of authorship fixed in a tangible medium of expression.
Copyright is automatic! If you have created an original work of authorship fixed in a tangible form of expression, it is automatically copyrighted by you, whether or not you register it.
If you want extra protection, you can register your copyright. A registered copyright is necessary for filing a lawsuit against an unlawful use of your work and can create extra security for your work, but costs money and isn't strictly necessary. For more information, look under the "For Composer's" tab on the left.
Copyright covers many genres of creation other than music. The Copyright Act has a few specific clauses for what parts of music are protected. More specifically, the Copyright Act protects the music, lyrics or libretto, and sound recordings of musical compositions. Performers also have their own specific rights. See the "For Performers" tab for more information.