Continental Drift
Around 1912, a German scientist named Alfred Wegener theorized that all of the Earth's continents were once joined together in a single, large landmass. He further proposed that the continents have separated and collided as they have moved around over the last few million years. He called this theory continental drift. He provided several pieces of evidence to support his theory:

1) Continent Shapes- The continents appear to be shaped in such a way that they would fit together nicely, like a jigsaw puzzle.

2) Rock Formations- There are rock formations on different continents that match up beautifully when the continents are put back together.

3) Fossils- There are fossils found on different continents that would also match up nicely if the continents were all once together.

People of the time mostly thought Wegener was crazy!

New Evidence
In the 1950's, scientists discovered some surprising evidence in support of Wegener's theory. While mapping the ocean floor, scientists discovered two important, and unexpected things:

First, the age of the rocks that make up the ocean floor gets older as you move away from the ridges at the center. This meant that the youngest rocks were found near the ridges, and the oldest rocks near the continents.

Second, there are stripes of alternating magnetic polarity on each side of the ridge. When the molten rock hardens, the magnetic minerals in the rock align themselves with the Earth's magnetic field. Scientists discovered that the Earth's magnetic field has reversed itself many times, at intervals of roughly every 100,000 years. The pattern they observed makes sense if the ocean floor is being formed at the ridge and gradually pushed outward in both directions.

These discoveries gave rise to the now respectable science of Plate Tectonics. This is the theory that the Earth's seemingly solid crust is actually made up of several pieces, or plates, that move around independently.

Plate Boundaries
The places where the different plates meet, called plate boundaries, are where the tectonic action really is. There are three basic types: convergent, divergent, and transform boundaries.

Convergent Boundaries: This a when two plates are moving toward each other, as shown above.

If the two plates are of relatively low, and similar densities, the plates will form a Collision Boundary.

In this scenario, the crust is forced upward by the collision, resulting in mountain building. The diagram above shows how this type of collision between India and China forced the formation of the Himalayan Mountains

If one of the plates is more dense than the other, as happens when oceanic and continental crust meet, then the more dense plate will be forced under the less dense plate. This forms a trench, or deep valley, where the plates meet. This is called subduction, and is shown in the diagram above. This often results in a chain of volcanoes running parallel to the trench.

Divergent Boundaries: As you might expect, this is essentially the opposite of a convergent boundary. This occurs when two plates are moving away from one another, as shown above. This is seen at mid-ocean ridges and rifts.

Transform Boundaries: This type of boundary forms when two plates are sliding past one another. The diagram above illustrates this motion. The most popular example of this is the San Andreas Fault in California.

All of the different boundaries and their locations are found on  page 5 of the Earth Science Reference Tables, shown below. Notice the key that shows the different boundaries and their symbols.

Tectonic Forces
The movement of the plates is driven by convection currents in the mantle. These currents cause the solid plates to float along on top of the semi-molten mantle material.

Sometimes, there is an opening in the middle of a plate that allows the molten material to flow through it. This is called a hot spot, and usually results in a chain of volcanic islands that form as the plate moves over the hot spot. The Hawaiian Islands are a great example of this.