Tectonics, also known as plate tectonics, is the theoretical understanding of how the surface of the earth is constantly shifting. According to the best understanding of science, giant tectonic plates are always moving very slowly as the Earth generates new crust and reclaims old crust. The effect has often been compared to a giant conveyor belt. There are certain points under the ocean where crust is generated, and these look like mountains, while other areas which look like trenches are the spots where the older crust segments are reclaimed. The understanding of plate tectonics is generally used to explain many geological occurrences on Earth, including earthquakes and volcanoes.
There are three main kinds of boundaries between different plates around the Earth. Some are moving toward each other, some are moving apart, and some are moving sideways next to each other. At the edges of these boundaries, things such as volcanoes and earthquakes are more common because the Earth is moving, leaving openings for magma to rise up and because the movement between the plates generates friction. A lot of the boundary areas are near the shoreline of the ocean, which is one reason why those areas are often more prone to geological activity. Studies have shown that the continents move at a speed roughly equivalent to 4 inches (about 10 cm) per year.
The whole idea of plate tectonics is a more advanced version of an idea developed in the early 1900s called continental drift. During that time, a scientist named Alfred Wegener decided to investigate some obvious facts that he found curious about the way the Earth was put together. There were certain similarities between the edges of continents where it looked as though they had broken apart at some point. Wegener started looking into the fossil records in these areas and discovered that there were surprising similarities, and he thought they warranted further investigation. For example, he found fossils of ancient plants and animals that were identical in areas that were separated by ocean.
Wegener started to believe that continents were moving around very slowly on the Earth's surface, and he even theorized that nearly all the land on Earth was once part of a giant single continent. Wegener's problem was that he could not explain how it was happening, and other scientists were very skeptical. In 1929, a scientist named Arthur Holmes came up with the basic idea for the currently accepted mechanism, but most scientists didn't really adopt the theory until the 1960s. Since that time, much evidence has been gathered to support the theory, and it is widely considered to be a fact.