Creative Commons
Creative Commons lets people share their work (photos, writing, etc.) with the world so that anyone can use and remix their creations under the licensing terms the authors provide. It is usually denoted with “Some Righs Reserved” as opposed to “All Rights Reserved.” Flickr has one of the world’s largest collections of such photos that people have shared with just such a license.
Most people get confused about the concept of Creative Commons. To get a better understanding you might want to read Skelliewag’s post about Flickr and Creative Commons licenses and how it all applies to your blog. It’s one of the best explanations out there.
The Photo Dropper Plugin makes the process of finding and adding these photos to your posts a lot simpler and takes care of necessary attribution automatically.
Here are some more articles that explain the consequences of licensing work under Creative Commons and the Wikipedia post about the topic.