Introduction to FDM：

Stratasys of Eden Prairie, MN makes Fused Deposition Modeling (FDM) machines. The FDM process was developed by Scott Crump in 1988. The fundamental process involves heating a filament of thermoplastic polymer and squeezing it out like toothpaste from a tube to form the RP layers. The machines range from fast concept modelers to slower, high-precision machines. The materials include polyester, ABS, elastomers, and investment casting wax. The overall arrangement is illustrated

Advantage:

Two build materials can be used, and latticework interiors are an option.
Filament of heated thermoplastic polymer is squeezed out like toothpaste from a tube.
Thermoplastic is cooled rapidly since the platform is maintained at a lower temperature.
Milling step not included and layer deposition is sometimes non-uniform so "plane" can become skewed.
Make rapid progress in past few years and be used widely.