Short answers, pulled from the story.
Plate tectonics is the scientific theory that Earth's lithosphere, the rigid outer shell of crust and upper mantle, is broken into a number of large tectonic plates that have been slowly moving for between 3 and 4 billion years. The plates typically move at zero to about 10 centimeters a year, and their boundaries are where earthquakes, volcanoes, mountains, and oceanic trenches form.
Earth's lithosphere is divided into seven or eight major plates, depending on how they are defined: the African, Antarctic, Eurasian, North American, South American, Pacific, and Indo-Australian, the last sometimes split into the Indian and Australian plates. There are also dozens of smaller plates, the eight largest being the Arabian, Caribbean, Juan de Fuca, Cocos, Nazca, Philippine Sea, Scotia, and Somali.
The three types are divergent boundaries, where plates slide apart and new crust forms through seafloor spreading; convergent boundaries, where plates move toward each other to form subduction zones or continental collisions; and transform boundaries, where plates grind past each other along transform faults. The San Andreas Fault in California is an example of a transform boundary with dextral motion.
Slab pull is most widely thought to be the greatest force, as cold, dense oceanic plates sink into the mantle at trenches and drag the rest of the plate behind them. Mantle convection driven by density variations and gravitational sliding away from ridges also contribute, while tidal drag from the Moon remains debated.
The meteorologist Alfred Wegener described continental drift in 1912 and expanded it in his 1915 book The Origin of Continents and Oceans. The idea was widely rejected for lacking a mechanism until seafloor spreading was validated in the mid- to late 1960s, with the theory of plate tectonics defined in papers between 1965 and 1967.
Earth's crust is soaked with water, which helps form the weak shear zones that plate tectonics requires. Venus is thought to lack active plate tectonics partly because its high temperatures leave little water for such weakening, and Mars, smaller than both, shows magnetic striping that failed a magnetic reversal test, leaving its tectonic past unproven.