Your self is your sense of who you are, deep down — your identity. When you let someone else know you well, you reveal your true self to them.
If the subject of your thoughts is you, you're thinking about your self — or, alternately, yourself. There are parts of your brain that make it possible for you to think about the concept of self, and schools of philosophy devoted to exploring why people have a unique sense of self. Psychologists also study the development of the self, or the beginning of self-awareness, in children. Self comes from the Old English, in which it means "one's own person."