Plate tectonics is the theory that explains the structure, composition and internal workings of the Earth on a worldwide scale, while continental drift refers to the theory that the continents all used to be connected in one megacontinent which scientists dub Pangaea. Both theories attempt to explain the movement of parts of the Earth's crust.
Plate tectonic theory teaches that the lithospheric level is split up into plates that move over the Earth. Where these plates meet are boundaries where volcanoes, earthquakes or both take place as the plates rub against one another, sometimes colliding quite violently. These borderlines are known as "fault lines," and earthquakes are most common along these lines on the surface of the planet. These plates do not just bump into each other; instead, they come together or even slide sideways against one another, so many possibilities for movement exist.
The theory of continental drift claims that today's continents drifted apart, splitting Pangaea into the larger number of land masses today. Looking at such points of intersection as the west coast of Africa and the east coast of South America, it is sometimes easy to see how these land masses would have fit together. While movement continues on the Earth's crust, it is extremely slow.