"The idea of universal access to research, education, and culture is made possible by the Internet, but our legal and social systems don’t always allow that idea to be realized. Copyright was created long before the emergence of the Internet, and can make it hard to legally perform actions we take for granted on the network: copy, paste, edit source, and post to the Web. The default setting of copyright law requires all of these actions to have explicit permission, granted in advance, whether you’re an artist, teacher, scientist, librarian, policymaker, or just a regular user. To achieve the vision of universal access, someone needed to provide a free, public, and standardized infrastructure that creates a balance between the reality of the Internet and the reality of copyright laws. That someone is Creative Commons." -- From the Creative Commons website.
Many writers, photographers, and creators have opted to share their creations with others through a Creative Commons license. While the works are still covered by copyright, the creators have relinquished some rights to make their creations available for wider use without express written permission.
Copyright law is intended “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” (U.S. Constitution, Article I, Section 8).
Note that copyright is for a limited time; copyright protection does expire and items become available in the public domain. (See the "Is It Protected by Copyright?" link below to see if the item you're interested is still covered.)
Permission must be requested before copying or using a copyrighted work, with a few exceptions. You may copy a few pages of a book for your personal use, for example, but you generally cannot copy an entire book. See the "Fair Use" link below for specific guidelines.