When a man says to an attractive woman at a party, "I'm a Cancer. What's your sign?", he is not only using an opening conversational gambit (and behaving just like Cancer), he is also talking about Sun signs.
What is a Sun Sign?
From our standard viewpoint here on Earth, the Sun travels around the Earth once each year. Within the space of that year, the Sun moves through all twelve signs of the zodiac, spending approximately one month in each sign. the sign that the Sun was traveling through at the time of your birth is your Sun sign.
The Sun is our most powerful planet. It gives us life, warmth, energy, and food. It is the force that sustains us on Earth. the Sun is also the most important and pervasive influence in your horoscope, and in many ways determines how others see you. The position of the Sun in your birth chart governs your individuality, your distinctive style, and your drive to fulfill your goals in life.
Of course, knowledge of Sun signs is not only useful in casual social meetings. It is invaluable in close relationships. If your Cancer husband nags and criticizes, you'll know it's just his way of showing how much he cares about you. If your ten-year-old Virgo son acts like a premature old lad, fussing about putting everything exactly where he wants it, you'll understand why he needs to have his possessions neatly arranged. Knowing how scattered and forgetful a Gemini can be, you will find it easier to forgive your Gemini lover when your birthday present shows up two days late.
Most important of all, an understanding of your own Sun sign will give you an added measure of self-knowledge, a deeper insight into the stranger that is yourself.