Material Extrusion devices are the most commonly available — and the cheapest — types of 3D printing technology in the world. You might be familiar with them as Fused Deposition Modeling, or FDM. They are also sometimes referred to as Fused Filament Fabrication, or FFF.
The way it works is that a spool of filament is loaded into the 3D printer and fed through to a printer nozzle in the extrusion head. The printer nozzle is heated to a desired temperature, whereupon a motor pushes the filament through the heated nozzle, causing it to melt.
The printer then moves the extrusion head along specified coordinates, laying down the molten material onto the build plate where it cools down and solidifies.
Once a layer is complete, the printer proceeds to lay down another layer. This process of printing cross-sections is repeated, building layer-upon-layer, until the object is fully formed.
Depending on the geometry of the object, it is sometimes necessary to add support structures, for example if a model has steep overhanging parts.
Deutsch 
English