The Earth's crust – the rigid, rocky outer layer of the planet that we live on – is composed of different tectonic plates that move across the surface of the Earth. When these plates collide, sometimes one plate will be thrust under the other, and forced down into the interior of the Earth. This process is called subduction.

Often, subduction zones occur where dense oceanic crust collides with less dense continental crust. The oceanic crust is forced underneath the continental crust. This process can lead to violent Earthquakes, such as the 2011 Tōhoku earthquake in Japan, and it can also form volcanoes.

Subduction can also occur in continental collisions, when two continental plates collide and one is forced underneath the other. Although the continents are generally buoyant and therefore resist subduction, the process of continental subduction is thought to occur at the intersection of the Indian and Eurasian plates.

A subduction zone occurs where you have one tectonic plate (generally oceanic crust) that gets pushed underneath another tectonic plate (either oceanic or continental).

Tectonic plates are not so much pushed underneath one another, as they are PULLED. As a plate is subducted under another, it goes deeper and deeper into the earth. As it moves, the pressure and temperature increase - this actually causes the minerals in the down going plate to change to minerals which are more stable at the new P-T conditions and have a higher density. The change in density is what pulls the plate down. As the plate is pulled down, water (trapped in minerals that are in the plate & the sediment on top of the plate) is released and rises into the bit of mantle above the down going plate, melting the mantle. The melted bit of mantle travels upwards, and results in volcanoes.

A great example of a subduction zone is our own Cascadia subduction zone. It extends from Canada (Mt Meager in British Colombia), all the way down to northern CA (Mt Lassen, which last erupted in 1915-1916). If you turn satellite view on in Google Maps, and zoom out so that you can see the Pacific Northwest, you can even see the outline of the Juan de Fuca plate (it is currently being subducted underneath the North American plate).

see image here

I hope that this better helps your understanding of subduction zones. They're really cool places where the Earth's crust gets recycled!

A subduction zone is a region in which the process of subduction occurs. Subduction describes the process in geology in which tectonic plates (pieces of the outermost layer of the Earth known as the lithosphere) collide, and one of the plates moves under the other, either due to the effects of gravity or is forced by the plate on top. The plate that moves under sinks into the mantle of the Earth, which is the portion from 700km to 3000km down from the surface (approximately). The subduction process occurs at convergent boundaries, where two or more tectonic plates undergo active deformation (distortion to their shapes).

Subduction zones are plate boundaries where one plate slides underneath the other and then subducts (sinks) down into the mantle. Normally this happens between a continental plate and an oceanic plate, but subduction can happen between two oceanic plates as well. Continental plates cannot subduct because the material of a continental plate is less dense than the mantle rocks underneath it, and so floats instead of sinks.

Here's a good description of a subduction zone - the biggest crash scene on earth! :

What Is a Subduction Zone? - Live Science Planet Earth

May 6, 2015 - A subduction zone is the biggest crash scene on Earth. These boundaries mark the collision between two of the planet's tectonic plates. The plates are pieces of crust that slowly move across the planet's surface over millions of years.

And we also have this:
Subduction-Wikipedia

From this article:
"Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle. Regions where this process occurs are known as subduction zones."

So, 'subduction' is the 'going under', and in a subduction zone, 1 plate will be going under, and 1 plate will be going on top - and I suppose they do a lot of 'crashing,' like the first link says.