All types of Equilibrium:
When we say equilibrium, it is a state of balance. Meaning the summation of all forces in a system is equal to zero and the three states of equilibrium are; stable, unstable and neutral.
Equilibrium has many different meanings, depending on what subject (chemistry or physics) or what topic (energy or forces). Dealing with energy there are three types of equilibrium.
Stable is when any sort of movement will heighten the object's potential energy. When objects in stable equilibrium are moved, they have a tendency to fall back to their original position. For instance, a skateboarder at the bottom, in the middle, of a ramp. Either way the skateboarder moves, his/her potential energy will increase because he/she will be raising in height. The boarder will also roll back to the bottom of the ramp if he/she doesn't exert any sort of energy to maintain the new position.
Unstable is when any sort of movement will lower the object's potential energy. When such objects are moved, they cannot return to their original position without some exertion of energy. For instance, when a coin is placed on its side, it exhibits unstable equilibrium. Any sort of push will cause the coin to fall flat, lowering its center of mass. The coin will not return to its side unless someone picks it up and resets it.
Neutral is when any sort of movement does not affect the object's potential energy. For instance, a ball on a table exhibits neutral equilibrium. If the ball rolls, the center of mass stays at the same height and thus it maintains the same equilibrium.