Creative Commons is a non-profit organization that created a set of simple, easy-to-understand copyright licenses. These are legally enforceable licenses that are meant to be simple to implement and easy for the average person to understand, and can be used to provide access to creative work in a standardized fashion.
The video below describes the origins and mission of the Creative Commons initiative:
Until the Copyright Act of 1976, the US government required creators to register for copyright protection. If the new creation was not registered, it automatically entered the public domain. Since 1978 (when the Act took effect), a new creation falls under copyright protection automatically, as soon as it is fixed in a tangible form. Furthermore, this is true even if the author does not want or need to reserve these rights.
Additionally, the copyright term has been vastly expanded since the first copyright law was established in the U.S. Constitution. In 1790, copyright protection was in force for 14 years, with an additional one-time term extension of 14 years possible if the creator took steps to secure it. Since 1978, the maximum term is the life of the creator + 70 years for single-authored works, or up to 120 years after creation for other types of authorship. The effect of these changes is to vastly reduce the number of works entering the public domain. The chart below illustrates the trend.