Technically speaking, your rising sign, or ascendant, reflects the zodiac sign that was ascending on the Eastern horizon at the moment you took your first breath in this world. This is why an exact birth time is so vital to finding your accurate rising sign. Located on the cusp of the first house of your astrological chart -- or at the nine o'clock position -- the rising sign can exert an influence almost as powerful as your Sun and Moon signs.
If you were to think of your Sun sign as your soul -- your inner personality and potential -- and your Moon sign as your heart -- your emotional core -- then you could say that your rising sign is your physical self or the face you present to the world. For example, though your Sun sign might be Taurus, your rising sign could very well be Gemini. While you're feeling laid-back, others will tend to see you as a ball of nervous energy and cerebral wit.
At times, your rising sign may act as a mask, hiding some aspect of your inner self you don't feel like revealing. Your Ascendant sign describes your temperament, your outward mannerisms and behavior, the personality you put on public display. It is the part of yourself you are most willing for other people to see in casual, impersonal situations. Changes here will explain why even people born on the same day as you will have very different personalities, different strengths and interests, and different life experiences.
The Ascendant is also called the Rising Sign of a person's chart. Astrologers use the two terms interchangeably. It is literally the sign that was rising on the Eastern horizon of the sky at the time you were born in the place where you were born. This is why it is essential to have both a birthplace and a birth time in order to cast a completely valid Natal Chart.