What is a copyright?
A copyright is a legal right the government grants to the author of an original work, such as a book or song, which gives the author certain exclusive rights in that work. In other words, for a specific amount of time, the owner of a copyright (generally, but not always, the author of the work) can control how his work is used, copied, displayed, etc., and prevent others from doing such things without the owner’s permission.
While the rights granted to a copyright owner are exclusive (meaning that the owner, and the owner alone, can exercise those rights), there are some predefined and limited exceptions. For example, one right granted to a copyright owner is the exclusive right to reproduce. Thus the owner of a copyright in a book is the only one allowed to make copies of that book. However, if you want to write a review of that book, you can copy a paragraph from the book to include in your review without infringing the copyright because this is fair use (one of the predefined exceptions to the exclusive copyright rights).