Simply put, a continental plate is a large block of "light" rock floating on top of the denser rocks that make up the Earth's interior. Because the minerals that make up the Earth's crust are less dense than those found in the mantle below, plates tends to rise to the surface of the Earth (much like ice rises to the top of a glass of water). Plates boundaries are defined by huge cracks in the Earth's crust called faults. Usually, the rocks on one side of the boundary are moving at a different speed and in a different direction from those on the other side. Sometimes, the plates are colliding with each other, forming big mountains (like in the Himalaya), other times, they are sliding along each other (like with the San Andreas fault in San Francisco). The Earth's surface is divided into 7 major continental plates, within which the rocks are generally moving as a singular body. For example, the Pacific plate is moving northwest at around 5-6 cm/yr. That means that in Santa Barbara (on the Pacific plate), we are moving about the length of your pinky finger toward Alaska every year!