• As a content producer: You have created a web page or a program and want to make sure that people use it the way you want them to.
• As a content user: You need a picture for your web site and are not sure where to find one that you are allowed to put online.

As a content producer, you can choose from an abundance of licenses. They even have names that sound partially familiar: Eclipse Public License, Apache License, BSD License, … But the licenses that are best explained for mere mortals are the Creative Commons Licenses: The summary looks as follows:
A separate page then explains what the icons mean. The site provides several ways of including a license: as a button for web pages, as a text file for file archives, etc. You can also choose your license in a wizard-style manner.
An important idea in software licensing is copyleft (the reversed C icon above): You don't exert any copyright over your work, but force derived work to do the same. The latter requirement has led to Microsoft calling copyleft “viral” (a term that has since lost its negative connotation).
So how about you wanting to use a picture? There, Creative Commons (CC) helps you, too: If you do an advanced search on Flickr, you can choose the CC license a picture should have. The same holds for the search options of Picasa Web Albums (when the results are displayed). I usually link back to the picture page underneath the picture or make the picture itself a hyperlink. Wikipedia pictures also carry a CC license. Lastly, the CC site has a search page to search for liberally licensed content.

From http://2ality.blogspot.com/