Pumps classified based on number of impellers:

Single stage pump:
A single stage centrifugal pump is the simple design of centrifugal pump which has one impeller mounted on the shaft within the casing and the fluid flow is regulated by the rotation of the shaft by motor.These types are used for higher flow rate and low pressure installations.

Multi stage pump:
In multistage pump the impellers mounted on the shaft is more than two. These are the pumps in which the impellers are connected in series on the shaft.
The multistage pump has four main parts:
Stator, rotor, bearing and shaft seal.

The stator section consists of the suction section, discharge section, and guide vane.
The rotor part is composed of the shaft, impeller, balance disc, and shaft sleeve.
Bearings are mainly composed of bearing bodies, bearings, and bearing glands.
The shaft seal is mainly composed of the sealing box body, packing, and water retaining ring on the water inlet section and the tail cover.

Difference between single and multistage centrifugal pump:
The single-stage pump has high flow and low pressure whereas the multistage pump has low flow and high pressure.
Single-stage has only one impeller whereas multistage has more than two impellers.
Single-stage can’t be used for high head applications but multistage can be used for high head applications.
Maintenance of multistage pumps is difficult than a single stage.