What is the difference between pair and joint?

pair: it generally refers to the two machine members which are coupled, together based on their relative motion

the types are turning pair, sliding pair etc.

example of turning pair is the piston and connecting rod.

example of sliding pair is the piston and the internal surface of the cylinder walls.

joint: it is the type of connection that exists between two machine members joined together, based on the relative motion existing between the members.

example: the joint between piston and connecting rod is pin-joint.