What is difference between piston and plunger?

1-in piston pump uses O-ring

2-in piston pump use metalic piston cylinder

3-piston pump use for expensive service

4-low pressure then plunger pump

1-in plunger pump use gland packing

2-plunger pump use for medium service

3-in plunger pump use in casing,cam and gijan pin

4-high pressure use 1000psi