Plate tectonics is the theory that explains continental drift. Tectonic plates are the pieces of Earth's crust and outer mantle. They include continental and oceanic crusts. Continental drift is the movement of these plates. And the theory of plate tectonics is the whole idea about it.
Continental drift is the name of the theory of the earth from around 1925 by Alfred Wegener who suggested it based on the fit of the continental coasts of the Atlantic Ocean, later based on evidence of fossil correlations between southern Africa and South America, but not adopted overall as a theory of geology.
Plate tectonics is the modern term used from the 1960's on after geophysicists learned the mechanism of sea-floor spreading at mid-ocean ridges and of earthquakes in the ocean trenches and were able to learn the facts about the division of the globe into lithospheric plates created and destroyed by the convecting asthenosphere, and were able to adapt this model as a framework within which geologic processes operate.
Plate tectonics, is the theory that the continents are on different plates, under the earth's surface. Continental drift, it the theory that the continents used to be one very large continent called Pangaea.
Continental drift is the theory that all the continents were connected together at one point and then over time plate tectonics spread and causing what was on the surface to move causing the contenents to move and move up to like an inch a year