The Earth's crust is the geological outer layer of the Earth. There are two types of crust: continental and oceanic.
Continental crust is made up of less dense materials than oceanic crust and is generally much thicker (generally 25-75km thick as opposed to oceanic 6km). The thickest crust is found underneath the Himalayan mountain chain where the roots of the mountains extend to around 75km.
Continental crust is also much older than oceanic. The oldest crust in existence today is found in Australia and has an apparent age of 4.4 billion years. Many believe that the Bible discredits the notion that the Earth is billions of years old.