When two continental tectonic plates collide, it causes the rocks in both plates to fold and eventually become piled up to form mountains. The most obvious example of this are the Himalayas, which were formed as a result of the collision between the Eurasian and Indian continental plates.
When a continental and an oceanic plate collide, the oceanic plate slides underneath the continental plate, which leads to the formation of volcanoes due to the rocks in the oceanic plate turning to magma as they are sucked deeper into the earth. This happens due to the fact that oceanic plates are denser than continental plates. However, continental plates are much too dense to be pulled into the earth, which is why the rocks rise to form mountains.
The collision between two continents eventually joins the two fully together, as the rocks in each continent basically become welded together by the pressure and force of the collision. This type of continental collision occurs when a sea or ocean between the two slowly shrinks until it fully disappears and the continents join together to form a large mountain range.
Before it collided with the Eurasian plate, the Indian subcontinent was actually an island located on its own tectonic plate. As of 2014, the Indian plate is still moving further north, causing the Himalayas to continue to rise at a rate of approximately an inch per year.