Conjunction is close approach between two orbiting objects.

Satellites may have to execute collision avoidance maneuvers and burn expensive fuel resources if the predicted trajectory brings debris too close for comfort.

Conjuctions are when the objects are within a certain distance (basically a safety margin, usually several kms), a collision is an actual strike.

The words are self descriptive:

Collision = con + laedere = strike together (two things strike each other)

Conjunction = con + iungere = join together (two things become joined to each other)