The most common type of hydraulic pump used in various applications is the "gear pump." Gear pumps are widely used due to their simplicity, reliability, and cost-effectiveness. They are positive displacement pumps that utilize the meshing of gears to create pressure and move hydraulic fluid.

There are two main types of gear pumps:

1. External Gear Pump: This type of gear pump has two external gears that mesh together. The fluid is carried around the outside of the gears and forced into the outlet as the gears rotate.

2. Internal Gear Pump: Internal gear pumps consist of an outer gear with external teeth and an internal gear with internal teeth. The fluid is carried between the teeth of the gears and is pushed to the outlet.

Gear pumps are commonly found in various hydraulic systems, including industrial machinery, automotive applications, construction equipment, and more. However, it's worth noting that the field of hydraulic technology is continually evolving.