When you betray someone or something, you provide information whether you mean to do it or not, like the loud growling of your stomach that betrays your hunger or the secret you tell about your friend that betrays her trust.
When you betray someone or something, you reveal something, like a secret or your true feelings. You may betray your impatience, for example, if you sit at your desk tapping your fingers. There is another meaning of betray that is just the opposite: lie. You betray your friend if you pretend that something is true just to trick him into doing what you want. In this case, betray means the same thing as deceive.