There are a large number of 3D printers that use “material extrusion” to print 3D objects. Material extrusion is any 3D printer that builds up a 3D object, layer by layer, by melting a material from a computer controlled nozzle. The material extrusion of thermoplastics was developed by a company named Stratasys. Stratasys named the technology “fused deposition modeling,” or FDM, and has trademarked the term. Since then, the term FDM has become widely use to generally mean the extrusion of thermoplastics. The company 3D Systems refers to the same technology as “plastic jet printing” (PJP). Other names for the material extrusion process are “fused filament modeling” (FFM), melted and extruded modeling” (MEM), “fused filament fabrication” (FFF), or “fused deposition method” (also FDM).
So, if you are looking to purchase a 3D printer, the terms FDM, PJP, FFM, MEM or FFF all mean the same thing: the printer produces objects by material extrusion of thermoplastics.