What is the difference between nuclear fission and fusion?

Answer 1

In fusion, two small atomic nuclei join to make a bigger one. In fission, a heavy nucleus breaks up into two smaller ones.

Answer 2

So which one is more energy efficient?

Answer 3

How do you define energy efficiency? Both convert mass to energy, according to the formula
E = m c^2, where m is the mass loss.

Fusion reactions that involve the lightest elements are more energetic than fission reactions.