What is the difference between mid-ocean ridges and trenches?

Ridges = crust formation
Trenches = crust destruction

Explanation:
Mid-ocean ridges are areas of rifting where two tectonic plates are diverging from one another. Magma rises up through the rifts and forms mountain ranges. The largest is located in the middle of the Atlantic. Rifting creates new crust.
Deep-ocean trenches are formed at convergent plate boundaries where the more dense plate (usually the oceanic) subducts under the less dense (usually continental). Subduction destroys old crust and is often accompanied by volcanoes. They are common along the pacific rim.