There is not a single extruder type, your choice will depend on the kind of 3D printer that you have, on the materials that you will use, and on the printing speed and accuracy that you need.
There are two different possibilities: Direct or Bowden extruders. The nature of your projects will determine which extruder you need to use. First, all extruders have motors, but there are also geared extruders to control your print speed. It is not essential, but it can help you to customize your setups in order to improve your print quality.
Direct extruders are directly attached to the hot end, while a Bowden extruder (or remote extruder) has a tube to link the hot end and the extruder body. For direct extruders, the gear rotates by a stepper motor driving directly the filament to the extruder hot end. The filament path is shorter, that is why Direct extruders are better to 3D print flexible materials than Bowden extruders You can totally 3D print flexible filament with a Direct extruder, but it is not really convenient.
With Bowden extruders you can 3D print your projects faster. It is easier to accelerate or decelerate because there is just the printing head to move, not the whole extruder hot end.
They both can have a direct drive system. Indeed, for both of them the filament drive mechanism can be mounted to the motor shaft, directly.
Moreover, keep in mind that there are extruders for all filament thickness, for 3mm filament, 1.75mm filament, etc.