When people ask, "What sign are you?", they are asking for your sun sign. The sun is the most powerful influence we have in our lives. Without the sun there is no life. (Please note: The sun is referred to as a planet for simplicity sake, but the acknowledgement of all other planets revolving around the sun is indicative of the sun's prominence in your horoscope)
The sun in your horoscope represents the inner YOU. It represents you as spirit. It is your inner light and deepest purpose as a "being" in a body.
Other planets in your horoscope add complexity and personality, but the sun is simplicity, spirit, and genuine in essence.
There is a wide range of simple expressions within any of the twelve signs, and no two people of the same sign will be exactly alike. They may even be so very much unalike. What they do have in common is the focus of the sign their sun shares. That is to say, for example, that the brave and courageous Aries sign will have it's share of quiet and shy types that, never-the-less, are dealing in their own way with the issues of courage and pioneering desires.
Because the sun represents our selves at our core it is wise to hold our judgements about people until we have looked deeper into their BEING, or as a spiritual teacher said often, "See the Being in the body."