The earth's surface is made up of crustal plates that float on top of liquid magma in the mantle of the planet. The plates are constantly being formed by liquid magma leaking up out of the sea floor, pushing the new crust away. The other edges of the plates are pushed up against each other, either pushing up to form mountains, or subducting below into the mantle, where the old crust is remelted and joins the liquid magma.
Sometimes the crustal plates rub against one another, creating pressure as they push together. If one suddenly slips and slides loose, this is what we call an earthquake. Imagine if you were pushing hard against your friend, and one of you slipped and fell. Both of you would lose your balance. The slipping of the plates releases a lot of stored-up energy, which reverberates through the surrounding crust. If an earthquake happens in the crust below the ocean, the buckling of the plates can create a tidal wave, which is what happened in Indonesia in 2004.
Last year, we had a small earthquake with an epicenter in Littleton. You may have felt a vibration or heard a loud bang. The reason that we had an earthquake is because we live near a small fault line, where two or more plates meet. Here is a great link on why we have earthquakes in New England:
Here is a picture of the San Andreas Fault in California. Now, that's a fault!