The zodiac signs
When you say "I am a Gemini" you are naming your Sun sign, the stretch of the zodiac the Sun was crossing on the day you were born. It is the best known piece of a birth chart, and the most misread: a sign does not describe a whole person, it describes a way of working.
Every sign comes from three things meeting. The element (Fire, Earth, Air, Water) tells you which material you work with: drive, substance, thought, feeling. The mode (cardinal, fixed, mutable) tells you what you do with that material: start it, hold it, or change it. The planet that rules the sign colours everything else.
This guide walks through the twelve signs one at a time. Each chapter shows you how the sign works when it works well, where it tends to trip, what it is really after, and how you recognise it in someone in front of you.
Read it with your own chart at hand. The Sun gives you a starting sign, but the Moon and the Ascendant change its colour, and in your chart several signs almost always speak at once.