Original: These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally.
Simplified: There are three types of plate boundary: constructive, destructive and transform. At a constructive plate boundary two plates move away from each other, and hot magma (liquid rock) is pushed upwards through the cracks. At a destructive plate boundary, two plates move towards each other. At a transform plate boundary, two plates move parallel to each other.