Darwin theory Watch
The other theory you should be expected to know at GCSE is Lamarck's theory, which states that characteristics were passed from parents to offspring, which is not true.
Darwin: variation, selection pressure, certain species don't progress, better adapted species continue.
Lamarck: Giraffe stretches to eat leaves, has a taller child.
Ultimately, the first explanation is better, (also, video is good for explaining the difference between evolution and natural selection so you can be petty and pedantic in your B1 exam!)