Copyright is a form of intellectual property rights. Copyright is granted automatically to authors/artists/creators as soon as they fix their original work in a tangible medium.
Under U.S. copyright law, copyright holders are entitled to the following 6 rights:
How long does copyright last? It gets a bit complicated depending upon when and where the work was produced (due to different copyright laws over time and by jurisdiction).
Current U.S. law automatically protects copyright on all works created after January 1, 1978 for the life of the author +70 years. For works for hire or anonymous works protections are for 95 years from publication or 120 years from creation, whichever is shorter.
For a detailed explanation with lots of scenarios, see Cornell Library's excellent guide to copyright term and the public domain or the U.S. Copyright Office's Circular on Copyright Duration.
In the United States, the purpose of copyright and other intellectual property law is laid out in the U.S. Constitution, where congress is given the power "to promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries" (art. 1, sec. 8). In other words, copyright is not just about benefiting creators, but about creating an environment where innovation can flourish for everyone. Granting exclusive rights preserves the ability of creators to be compensated for their creative work, while doing so for "limited times" and in order "to promote the progress of science and useful arts" creates room for categories like the public domain and fair use.
Aside from caring about copyright's broad goals and the ethics of fairly crediting and compensating others for their work, there are a wide range of potential legal penalties for copyright infringement, including paying actual damages, statutory damages ranging from $200–$150,000 per infringed work, lawyer's fees, and even criminal charges. While this should not be understood as a deterrent to legitimate fair use, it is a good reason to understand copyright law.
Information on this guide is for general informational purposes and should not be construed as legal advice. If you have specific legal questions about copyright, consult legal counsel.