"Essentially, a von Neumann probe is a small ship that contains all of the information it needs to recreate itself once it reaches its destination. Acting like a virus, a von Neumann probe would land on a distant planet or asteroid and begin to gather the material needed to build a replica of itself. Once the first replica was complete, it too would begin the same process, exponentially growing our fleet of explorers."

http://www.engineering.com/3DPrinting/3DPrintingArticles/ArticleID/6319/3D-Printing-the-One-Hundred-Year-Starship.aspx