Many different methods are used to classify pumps. Terms such as positive displacement, non positive displacement, fixed delivery, variable delivery, low pressure, high pressure and other terms used to describe the pump. The first two terms are the fundamental terms to describe the pump. That is, all pumps are either positive displacement pumps or non positive displacement pumps. In other words, the method that is used to supply the energy to the liquid determines the category into which the pump is placed.

So, there are two main categories of pumps:

Pumps duties may be range from a low pressure of a fraction of a bar and small capacities of a few litters per minute to a high pressure of several hundred bars and large capacities so, each of those categories has sub-groups consisting different types of pumps as shown in the following diagram:

Basically, pumps that discharge (pump) liquid in continuous flow are referred to non positive displacement pumps (Roto-Dynamic pumps) and those pumps that discharge liquid separated by a period of no discharge are referred to positive displacement pumps.