What are tectonic plates?

In geology, the Earth's outermost layer is called the lithosphere, which contains the crust and solid, rigid rock. Below the lithosphere is the asthenosphere, which flows more like liquid. The Earth's lithosphere is broken up into tectonic plates — massive, irregular slabs of solid rock that are assumed to average about 70 miles thick. The Earth consists of seven major tectonic plates (underneath the continents as well as the oceans) and many more, smaller ones. These plates float, or "ride," on the asthenosphere.
Tectonic plates sometimes collide. The world's great mountain ranges were formed when plates collided and the rock had nowhere to go but up. Tectonic plates are moving slowly, rubbing against each other or even in opposite directions, which is what causes the majority of the earth's seismic and volcanic activity. If you study where the tectonic plates are located, you'll notice that the vast majority of mountain ranges, volcanoes, and earthquakes occur along the edges of these plates, while the centers of the plates remain relatively stable, in geologic terms.