1. Strong Nuclear Force: This force is the strongest of the four and acts over very short distances, on the order of the size of an atomic nucleus. It is responsible for holding protons and neutrons together in the nucleus of an atom.

2. Electromagnetic Force: This force is responsible for all electrical and magnetic phenomena. It acts over long distances and is responsible for the attraction between opposite charges, repulsion between like charges, and the interaction of magnets.

3. Weak Nuclear Force: This force is responsible for radioactive decay and is weaker than the strong force but stronger than the gravitational force. It acts over very short distances, on the order of the size of an atomic nucleus.

4. Gravitational Force: This is the weakest of the four fundamental forces, but it acts over very long distances. It is responsible for the attraction between any two objects with mass.

These four forces are thought to be fundamental because all known physical interactions can be explained in terms of them.