The extruder forces the Filament into the hot end. It usually grips the filament by pinching it between a roller bearing and a knurled, hobbed, or toothed pinch wheel. Torque is usually supplied by a Stepper Motor.
Lifetime and maintenance
The filament sliding through the extruder will eventually sand down a track somewhere. Designs where friction is created between filament and extruder exist. An example of this is the Ormerod extruder design. It bends the filament as a spring to force filament away from the roller bearing and towards the hobbed bolt to achieve better grip. Its extruder body needs replacement every ~1000 hour of operation.
Most designs use PTFE (or sometimes PEEK) tubing to guide the filament. It has proven to be very wear resistant and have constant properties over time.