Leak vs Leakage?

The word "leak" can be a noun or a verb, but "leakage" is only a noun, so that's one difference.

When used as nouns the two words can have the same meaning, i.e. "an act or instance of leaking" or "any means of unintended entrance or escape."

The word "leak," however, can also have another meaning which is not relevant to the word "leakage" and that meaning is "an unintended hole, crack, or the like, through which liquid, gas, light, etc., enters or escapes."

The word "leakage" also has another meaning which is not relevant to "leak" and that is "something that leaks in or out."

As someone wrote on this forum thread, put very simply: "a leak is a hole, the leakage is the water that runs through it."