The term rapid prototyping (RP) covers a variety of processes aimed at quickly creating physical parts from 3D computer models using computer controlled machines. Virtually all RP processes are additive. The part is built up by adding, depositing, or solidifying material. This is the opposite strategy of a subtractive process such as machining. Machining creates objects by removing unwanted material from a large block in the form of chips. All current RP processes build up the object in a series of horizontal layers of varying thickness.
The word rapid in RP is a relative term, as most of these processes are actually relatively slow. Rapid actually refers to the reduced time from design to final part due to the elimination of extensive amounts of handwork (and setup/computer programming in the case of CNC machined parts) involved in making prototypes, as well as the ability to quickly iterate a design through various stages. This results in drastically shorter times to production.
A good synopsis of rapid prototyping technology can be found here.
For pretty much all Rapid Prototyping processes, one needs to start with a closed, valid polysurface or solid.
Information on producing valid closed solids in Rhino.
From that closed solid, a mesh representation of the model needs to be created and exported in the form of an .stl file.
SLS (Selective Laser Sintering)
FDM (Fused Deposition Modeling - Stratasys)
Polymer ink printer (Objet)
Solidscape and other wax printers
LOM (Laminated Object Manufacturing)