What happens when 2 tectonic plates collide?

When two plates collide directly (versus sliding along each other) they typically form a mountain range. For example, the Himalayan Mountains are are formed by the collision of the Eurasian Plate and the Indo-Australian Plate. As John McPhee says in "Annals of the Former World", the most amazing thing is that the summit of Mount Everest, the worlds tallest mountain, is marine limestone.