Pumps transfer liquids from place to place. Positive displacement pumps can be classified as rotary or reciprocating. Reciprocating pumps are characterized by a back-and-forth motion, whereas rotary pump move in a circular rotation. Dynamic pumps can be classified as centrifugal or axial. The centrifugal pumps use centrifugal force to move liquids ; the axial pump pushes liquids along a straight line.