Examples of external forces include the force applied to the system, air resistance of an object, force of friction, tension and normal force. Internal forces include the force of gravity, spring force, and magnetic and electrical field forces. Forces are either internal or external.
Internal and external forces are cornerstones of Newtonian mechanics and the laws of motion. When a force is applied to a system, it is external if it causes the overall mechanical energy, made up of both kinetic and potential energy, to change by either losing or gaining energy. External forces are also known as nonconservative forces. Kinetic energy is the energy caused by the movement of the system, and potential energy is the energy the system can use. The total energy of the system is the sum of the two.
The force is internal if the overall mechanical energy of the system doesn't change and is conserved. For example, when a ball is dropped from an elevation, gravity is pushing it to the ground. As the ball falls, potential energy is converted to kinetic energy as the ball goes from rest, where kinetic energy is zero, to being in motion. The overall sum of the system stays the same. Internal forces are also called conservative forces.