An external force is a force that originates from an object outside of a defined system. For example, if a system is defined as only the bob in a pendulum, both the rope and gravity exert an external force on the bob. If the bob and cord are the system, the force of gravity on the bob and the force the ceiling exerts on the bob are external forces.
In physics, the system is defined as the portion of the universe under consideration, explains a Western Washington University website. The system can either be a single object or a group of objects, but these objects do not need to be bound together, or even interacting with one another. Everything outside the system constitutes the system's environment and is generally ignored except for its effect on the system. Contrary to external forces, internal forces are forces that originate from within the system itself. From the previous example of the pendulum, if the bob and the cord are considered the system, the force of the rope on the bob becomes an internal force, as opposed to when the bob is the sole object of the system and the force from the rope is external. Consequently, if the earth was taken as the system, the gravitational force, ceiling force and the force from the rope would be considered internal, and the external force would be from gravitation with the moon and the sun.