They are determined by their direction relative to one another.
Divergent: The plates are moving away from each other. This results in plate margin ridges. Example: The Mid-Atlantic Ridge.
Convergent: The plates are moving towards each other. This results in mountains and subduction zones. Example: Ocean trenches and chains of volcanoes. The Andes Mountain chain and off-shore ocean trench.
Transform: The plates are sliding against each other in opposite directions. This results in major fault systems. Example: The San Andreas Fault in CA.