As soon as you create an original work of authorship and put it into tangible form, it is protected under common law copyright law. If you compose an original song, for example, it is copyrighted as soon as you record it. To sue someone for copyright infringement, however, you must first register your copyright with the U.S. Copyright Office.
You don't have to register your copyright before it is infringed to enforce your copyright. If someone infringes your work, you can register your copyright after the infringement occurs and then file a copyright infringement lawsuit. Your work doesn't even have to be published to be eligible for registration. You cannot sue for copyright infringement in a state court, however. Copyright law is federal law, and only federal courts will hear copyright infringement lawsuits.
If you registered your copyright before the act of infringement occurred, or within three months after you first published it (even if you register after the act of infringement), you may seek statutory damages -- that is, damages spelled out in the copyright statute. The main advantage of statutory damages is that you don't have to prove you suffered any losses. Statutory damages range from $750 to $30,000 per infringement. They can increase to as much as $150,000 per infringement in cases of willful infringement, which means the infringing party knew or should have known that your work was copyrighted. If you didn't register your work in time, you can still ask for the amount of your losses plus any extra profits made by the infringer.
Attorney's fees for a copyright infringement lawsuit can be so expensive that a copyright holder might not be able to afford to file a lawsuit even with a strong case. In complex cases, attorney's fees can exceed $1 million. Because of this, federal law allows you to recover reasonable attorney's fees in addition to any other amounts you may be entitled to. The possibility of an award of attorney's fees may convince an attorney to take your case even if you can't afford to pay anything in advance.
Copyright registrations are publicly available. If your work is registered before infringement and you file a lawsuit, you can seek to increase your damages by claiming that the defendant willfully infringed your copyright. Although the defendant might prove that he didn't actually know of your copyright at the time of the infringement, it would be difficult for him to argue that he shouldn't have known, because he could have easily checked with the U.S. Copyright Office.