There are four fundamental forces in nature, namely, electromagnetism, weak nuclear force, strong nuclear force and gravity. Electromagnetism can be observed in daily life experience. For instance, lighting is a natural phenomenon. Weak nuclear force is manifested in radioactivity, which is used to cure cancer. Strong nuclear force, on the other hand, is responsible for keeping the nucleus (protons and neutron) together. Whereas, the gravity is observed in everyday life, for example, the planets of solar system rotate around the sun due to gravity.
It is important to note that electromagnetism and gravity are long range forces, i.e., these forces can act for longer distances, whereas, weak and strong nuclear forces are only manifested at very short distances, i.e., of the size of a nuclei which is about 10-15 m. Gravity is very weak as compared to other three fundamental forces and it only manifests itself on massive objects. Since the mass fundamental particles like electrons or quarks (which are the building blocks of protons and neutrons) are very small, therefore, they don’t feel gravity as compared to the strength of others three forces. Hence in the study of the properties of elementary particles, physicists normally don’t consider the gravity, therefore, we are left with three forces, electromagnetism, weak and strong nuclear forces.