1. Nuclear Fission: This is the process where a heavy atomic nucleus (like Uranium or Plutonium) is split into two or more lighter nuclei. This process releases a tremendous amount of energy and is the principle behind nuclear power plants and atomic bombs.

2. Nuclear Fusion: This is the process where two or more atomic nuclei combine to form a heavier nucleus. This process also releases a huge amount of energy and is the source of energy for stars and the sun.

These are the two main types of nuclear reactions. However, there are other subcategories within these, such as neutron capture, alpha decay, beta decay, etc.