Rapid prototyping takes virtual designs (from computer aided design (CAD) or from animation modeling software), transforms them into cross sections, still virtual, and then creates each cross section in physical space, one after the next until the model is finished. It is a WYSIWYG process where the virtual model and the physical model correspond almost identically.
In additive fabrication, the machine reads in data from a CAD drawing, and lays down successive layers of liquid or powdered material, and in this way builds up the model from a series of cross sections. These layers, which correspond to the virtual cross section from the CAD model, are glued together or fused (often using a laser) automatically to create the final shape. The primary advantage to additive construction is its ability to create almost any geometry (excluding trapped negative volumes).
The word "rapid" is relative: construction of a model with contemporary machines typically takes 3 to 72 hours, depending on machine type and model size. Used in micro technologies "rapid" is correct, the products made are ready very fast and the machines can build the parts in parallel.
Some solid freeform fabrication techniques use multiple materials in the course of constructing prototypes. In some cases, the material used for the final part has a high melting point for the finished product, while the material used for its support structure has a low melting point. After the model is completed, it is heated to the point where the support material melts away, and leaving functional plastic prototype. Although traditional injection molding is still cheaper for manufacturing plastic products, rapid prototyping may be used to produce finished goods in a single step.
However, there are currently several schemes to improve rapid prototyping technology to the stage where a prototyping machine can manufacture its own component parts (see RepRap Project). The idea behind this is that a new machine could be assembled relatively cheaply from raw materials by the owner of an existing one. Such crude 'self-replication' techniques could considerably reduce the cost of prototyping machines in the future, and hence any objects they are capable of manufacturing.