Constructive margins are where two plates are moving away from eachother. Magma rises from the mantle to fill the gap, and cools. It creates a new crust
Conservative margins are where two plates are moving sideways past eachother, or are moving in the same direction, but at different speeds. Crust is ot created or destroyed.
Destructive margins are where two plates are moving towards eachother.
Where an oceanic plate meets a continenal plate, the denser plate is forced down into the mantle and is destroyed. This is often where volcanoes and ocean trenches are created
Where two continental plates meet, the plates smash together, but no crust is destroyed. (Fold Mountains)