What is formed when one tectonic plate slide under another tectonic plate?

Depends on the type of tectonic plate. If it is two continental plates, usually a mountain range will be formed and the plates will 'lock' together due to their thickness. The Indian sub-continent colliding with the eurasian plate is one example. If an oceanic plate is subducting under a continental plate an island arc can be formed or mountain range such as Japan and the Andies mountains respectively. This type is also where volcanoes will be commonly found unlike the collision of two continental plates, due to the magma formed from the subduction of the oceanic plate. If two oceanic plates collide, usually form an oceanic mountain range and lock together similar to the continental plates.