Free clip art can be found at OpenClipArt.org and Clker.com. These images are all free for personal and commercial use, which means you can use them however you like. However, you should still read the About sections of these websites to make sure there are no caveats to that.
If the clip art is for educational purposes, more free sites are available, such as ClasroomClipart.com and ClipArt ETC by the Florida Center for Instructional Technology.
Consumers who use clip art in projects should be careful to know how the image they're using is licensed, as the artists who create the digital images often retain the copyright for the clip art and have rules about how the images may be used. This includes images that are public domain works that were scanned by a third party; in being digitized, they are now derivative works, and if the person or organization who digitizes them does not give up the copyright for the image, it is subject to the same rules as if the person had drawn it himself.
Though the term clip art most commonly refers to digital illustrations, it pre-dates computers and was originally used to describe cutting and pasting physical pictures for use in other publications. The first widely available digital clip art library was on the IBM PC, through VCN ExecuVision in 1983.