A positive displacement pump works by trapping a specific amount of fluid inside an area of the pump before forcing the fluid to exit the pump through an outlet of some kind. The mechanism that traps the fluid selectively blocks either the entrance or exit when the fluid is moving.
The most common type of pump is the reciprocating pump. The reciprocating pump consists of a mechanism that consistently expands and contracts the cavity of the pump. The fluid simultaneously blocks the entrance and leaves the pump through the exit when the pump mechanism contracts and simultaneously blocks the exit and flows in through the entrance when the pump mechanism expands. This type of pump can accommodate small, fragile solids moving through it.
The next type of pump is the rotary pump. The rotary pump consists of rotating parts in a small cavity moving the fluid from one location to the next. The rotary pump can't accommodate much in the way of solids moving through it due to the necessities of the system.
The last type of pump is the linear pump. A chain attached to several discs is submerged in the fluid for this type of pump. As the chain and discs are pulled up through a tube, the fluid becomes trapped between the discs and is discharged at the top.