Creative Commons offers authors, artists, scientists, teachers and students the freedom to handle their copyright in a more flexible way. The author (person having copyright) decides, making a choice out of six (freely) available standard licenses, to what extend his or her work will be allowed to be spread and under which conditions this is allowed.
Offering your material under a Creative Commons license implies that the user gives up his or her copyright. This means that he/she chooses to offer any user certain rights, rights which the user otherwise would have to ask for explicitly.
Here you can find an overview of the six most important Creative Commons licenses.
Material that has been published with a Creative Commons license is recognizable by this sign: