3D printers are the simple version of rapid prototyping machines. It is lower lost and less capable.
Rapid prototyping is a conventional method that has been used by automotive and aircraft industries for years.
In general 3D printers are compact and smaller than RP machines. They are ideal for use in offices. They use less energy and take less space. They are designed for low volume reproduction of real objects made of nylon or other plastics. That also means 3D printers make smaller parts. Rapid prototyping machines have build chambers at least 10 inches on a side, a 3D printer has less than 8 inches on a side. However a 3D printer is capable of all the functions of rapid prototyping machine such as verifying and validating design, creating prototype, remote sharing of information etc.
Consequently 3D printers are easy to handle and cheap to maintain. You can buy one of those DIY kit in the market and build up yourself. It is cheaper than the professional rapid prototyping, for $1000 or less you can have one 3D printer. While the professional rapid prototyping cost at least $50,000.
3D printers are less accurate than rapid prototyping machines. Because of its simplicity the material choices are also limited.