A leak involves the release of something. It can be a behind-someone’s-back leak of sensitive information, a leak from a milk carton, or a leak in the roof that lets rain water drip in.
As a verb, leak means to allow light or fluid to escape, or to enter or escape as though through a hole. Light can leak in through the curtains, water can leak out of an almost-closed faucet, or information can leak to the press. A leak is almost always a bad thing — it indicates that something that should be contained has broken out or escaped.