The rigid Lithosphere (crust + upper mantle) is broken into plates that float atop the flowing, semi-liquid Asthenosphere.

Magma rises from the mantle. Forms Mid-Ocean Ridges (seafloor spreading) or Continental Rift Valleys. Creates new crust.

Subduction: Dense oceanic crust sinks below continental crust, melting and forming coastal volcanoes and deep trenches.
Collision: Two continental plates crush together forming massive folded mountains (Himalayas).

Plates slide past each other. No magma, no new crust. Massive tension builds and releases as Earthquakes (e.g., San Andreas Fault).