I've been looking into this as well. Copyright claim is the big thing I think. Legally, a work is copyrighted at the moment of creation. After that, it's all about being able to prove it. From US copyright office-
Copyright Protection Is Automatic
Under the present copyright law, which became effective January 1, 1978, a work
is automatically protected by copyright when it is created. A work is created
when it is “fixed” or embodied in a copy or phonorecord for the first time.
Neither registration in the Copyright Office nor publication is required for
copyright protection under the law.
Advantages to Copyright Registration
There are, however, certain advantages to registration, including the establishment
of a public record of the copyright claim. Copyright registration must
generally be made before an infringement suit can be brought. Timely registration
may also provide a broader range of remedies in an infringement suit.
Ascap BMI, etc. are organizations that exist to collect fees for your musics use, and to enforce the copyright.
Since my band is not huge, and really exists only in the local scene, I am not to worried about needing to collect money from Steven Speilberg because he ganked our songs. Not yet, anyways. I went ahead and submitted a copyright claim to the us copyright office
http://www.copyright.gov/
and I figure down the road, if it comes to that, I can register with the various organizations if needed.
I'm still navigating the waters and figuring this all out, so would love to hear if I have any of this wrong or if there are other things.