Gear pumps use meshing gears to pump fluids through displacement. There are two main types: external gear pumps which use two external spur gears, and internal gear pumps which use one external and one internal spur gear. Gear pumps are positive displacement pumps that pump a constant volume of fluid per revolution. External gear pumps are commonly used for lubrication and fuel transfer and internal gear pumps can handle high viscosity fluids like asphalt or chocolate. Both have advantages like precision and constant output, but also limitations like fixed flow rates and moderate pressure ranges.