Oceanic crust is the layer of rock at the bottom of the ocean. It is formed when rock from the mantle melts and comes up at mid ocean ridges. As more crust gets formed, the older crust is pushed away from the ridges and eventually goes back down into the mantle at trenches near the edges of continents.
Since oceanic crust is constantly being recycled, it doesn't get much older than 200 million years. By comparison, continental crust can be billions of years old. Oceanic crust is made mostly of basalt and gabbro and is about 7 km (4 miles) thick. Continental crust, on the other hand, is often five times thicker.