A universal constructor is a machine that can replicate itself and – in addition – make other industrial products. Such a machine would have a number of interesting characteristics, such as being subject to Darwinian evolution, increasing in number exponentially, and being extremely low-cost.
A rapid prototyper is a machine that can manufacture objects directly (usually, though not necessarily, in plastic) under the control of a computer.
The RepRap project is working towards creating a universal constructor by using rapid prototyping, and then giving the results away free under the GNU General Public Licence to allow other investigators to work on the same idea. We are trying to prove the hypothesis: Rapid prototyping and direct writing technologies are sufficiently versatile to allow them to be used to make a von Neumann Universal Constructor.