With beams, airfoils, solid bodies, and more, they all experience external and internal forces. The main difference between these two forces is an external force is relative to the external coordinate system and an internal force is relative to the internal coordinate system. Let's look at an example of this to better understand this difference!
Let there be a beam attached to a wall like below.
Fig. 1 - Image of a beam with external forces.
The beam is attached to the wall with point A being to the far left of the beam and with points B, C, and D spread across the beam with point D being on the far right. Each of these points have forces applied to them. For example, point D has a force of 2P pointing to the right and point B has a force of 4P pointing to the left. Now all of these forces are examples of external forces because they are relative to the external coordinate system.
Now, if we say the force distribution internally to the beam is linear, then we find what the internal forces are between the points. Let's take a section cut between points B and C to see the internal force.
Fig. 2 - Image of the linearly-distributed internal forces within a beam.
In Figure 2, it shows the internal force, $ F_{BC} $, between the points B and C. This force is the same between B and C because we had said the forces are linearly-distributed throughout the beam. Generally, a good way to tell the difference between an internal force and an external force is the relative coordinate system that is being used. For example, we know the force $ F_{BC} $ in Figure 2 is an internal force because we had to take a cross section to analyze the forces inside of the beam, whereas in Figure 1, all of the forces shown are external.
Now we have learned the difference between an external and internal force!