The function of copyright, as its name implies, is that it guarantees for the creator of a work the rights to copy, reproduce and distribute it. In the U.S., this guarantee is rooted in the Constitution. However, from its legal origins (in England in 1710 and in the U.S. in 1790) copyright was as much about the promotion and circulation of knowledge and good ideas as it was about the protection and rewarding of creators. Limitations and exemptions to creators' copyright protection were as important to society as the protection itself.
Copyright protection applies not to facts nor to ideas but to the particular expression of ideas in a tangible medium. The duration of copyright protection is for the lifetime of a creator plus 50 years (although legislation is before Congress to extend that protection for a further 20 years). Creators have the exclusive rights to reproduce their work, to prepare derivative works, to distribute copies, and to perform and display their work publicly. Creators can and usually do assign their rights to others for commercial exploitation of their creations.
The rights of copyright holders are spelled out in Section 106 of Copyright Law (Section 106 of Chapter 1 of Title 17 of the U.S. Code, to be precise).