There are two types, Bowden and Direct Driver extruders. Bowden extruders are located away from the printer head and push the filament from a distance into the hot end. As it is not located above the hot end, it means it does not move with the print head making for more accurate prints. Direct Drive extruders are located directly above the hot end and pull the filament into the hot end from a short distance away. This in theory reduces thing from going wrong when feeding, but does add more weight to the moving printing head that can compromize print accuracy, especially during sudden changes in direction.