Copyright exists from the moment you create your work and fix it in tangible form. However, even though it is not required, registering your photo with the US Copyright Office gives you the full protection of the law.
If you register BEFORE your copyright is infringed upon, (or within 90 days of publication), you can sue for statutory damages ($750 to $150,000) and legal fees. If you don't register, you can only claim what your work MIGHT have sold for, which is hard to prove.
To register, go to the website:
http://www.copyright.gov
where you can register online or download an application, and submit your work. It costs $35 online, or $50 to $65 through the mail. On a single application, you can register unpublished work as a collection, as long as it is in the same year as the application. So, you only have to pay the fee once for a big batch of photos.
It takes from two to six months, (we are dealing with the government here), to get your certificate of registration, - unless you choose to expedite it, .... which costs an additional $760.... (see what I mean about dealing with the government).
There is also a Creative Commons license (CC) which lets you share photos without losing copyright while you specify clearly how they are to be used and that you are always credited. A CC license does not give you the same protection as a registered copyright, so only apply for a CC after you have registered. Here is more info on CC:
http://creativecommons.org
Be aware, there are lots of uses that don't necessarily violate copyright. For instance, copyrighted photos can be used without permission or payment "for purposes such as criticism, comment, news reporting, teaching, scholarship, or research". Many photo contests have rules allowing the sponsor to use entries for promotional and other purposes. You retain copyright, but you might not get paid.
Here is another not always widely known fact: If you shoot a photo for your employer, or under a work for hire contract, your EMPLOYER owns the copyright!
This information is taken from the book "The Law (In Plain English) for Photographers" by attorney Leonard D. Duboff.
Copyright is a LEGAL issue, and thus is open to many and varied interpretations and laws, .... that is why there is such a thing as copyright lawyers. When in doubt about something concerning your copyright, you have to seek expert legal advice.
steve