Once a work is "fixed" in a "tangible medium of expression," it is automatically protected by copyright. Registration is not necessary to receive copyright protection. An unregistered copyright entitles you to reproduce, sell, and perform the copyrighted work. Beyond this, your rights are limited if your copyright is not registered, especially if you have distributed your work through a copyleft agreement.
You cannot bring a suit for copyright infringement without first registering your copyright. It is possible to sue for an injunction, which would order the individual using your copyrighted work to stop using it.
It is possible to get actual damages and profits in a suit involving an unregistered copyright. Actual damages and profits are the actual money lost (or potentially lost) by the copyright owner and the money gained by the individual using the work. If the copyright is registered, it is possible to receive statutory damages (damages called for by a statute) as well as attorney’s fees.
Because of the limited rights you have with an unregistered copyright and the many benefits to registering your copyright, you should strongly consider registering your copyright.
Defending your copyright, whether registered or not, can be very difficult. An experienced intellectual property attorney can guide you through the process of registering a copyright and ensure that all deadlines and requirements are met. If you need to sue or are sued for copyright infringement, an IP lawyer can represent you in court.