The yolk (sun) is left completely intact and stays mostly uncooked. The white of the egg is cooked most of the way through and usually is left with just a little bit of sheen to it.
The yolk is left intact. The egg is flipped once and cooked only for a few minutes. The white of the egg is cooked through while the yolk is still runny.
Over-medium eggs are cooked the same as over-easy because they are flipped while frying, but are just cooked a bit longer. The yolk in an over-medium fried egg is only slightly runny.
The yolk is cooked completely through, similar to a hard-boiled egg. The egg is flipped while it is cooked, and it is done longer. Both the egg white and yolk are cooked until they are considered done and nothing is raw.
There are so many ways to cook fried eggs. Sunny-side-up, over-easy, over-medium, over-hard. But do you know the difference?