Zodiac Signs are 12 segments of the ecliptic (Sun's path), named after 12 constellations that extend across the ecliptic. Each sign is associated with a particular set of qualities, characteristics, events and objects.
The 12 signs are typically of equal length – 30°, despite the fact that the size of the corresponding constellations vary dramatically.
Zodiac signs are based on a complex yet elegant system that goes far beyond what constellation they happened to be associated with. There are 12 signs, which divide into 4 elements or 3 modalities or 2 polarities. There are geometric relationships between these sign qualities that correspond to the aspects.
The 7 traditional planets (Sun through Saturn; visible to the human eye) are each associated with one or more sign as its ruler or ambassador, which is also part of astrology's complex yet elegant system.
When you say you're a ___{your zodiac sign}___, it means the Sun was travelling through that sign when you were born. True astrology looks beyond the Sun sign to where all standard planets and bodies were at the moment of someone's birth.
Recent suggestions (primarily by non-astrologers) that the astrological zodiac system "should" include other constellations that cross over the ecliptic (e.g. Ophiuchus) are based on a lack of understanding of the fundamental premises of astrology.
