Alfred Wegener suggested that around 200 million years ago, all the continents were joined together to form one super-continent. He called the super-continent Pangaea. Then over millions of years this broke up and the continents moved apart. He called their movement continental drift.
The evidence Wegener used to prove his theory of continental drift was: 1) Fossils - Animals and plants only survive in habitats that suit them yet fossils of these have been found on different continents, often in completely different climates e.g. the small reptile Mesosaurus lived 240 million years ago and swam only in shallow waters. Its fossils have been found in Brazil and South Africa, suggesting that these were once joined. 2) Rocks - Rocks of the same age, composition and structure have been found thousands of miles apart 3) - The shapes of continents - the continents look like pieces of a jigsaw puzzle so if you move them around, they will fit together. This is known as continental fit.
Sea-floor spreading is the process whereby molten basalt is continually welling up at oceanic ridges forming new rock. At the same time the older ocean crust is moving aside, carrying the continents with it.
The evidence for sea-floor spreading is that the rock in the ocean floor is much younger than the rock in the continents and the further it is from the oceanic ridges, the older the ocean rock.