The Latin term Zodiac translates to 'zoo of animals'. This belt is one of the great circles of our astronomical heavenly vault. It is the path that our Sun takes (also called the ecliptic) in our galaxy that is dotted with 12 constellations, one for each of the signs of the Zodiac. Many of these constellations are animals, like the Crab, the Ram, the Lion and the Archer, while a few are people such as the Gemini twins, and Virgo the virgin.
There are basically two systems to determine the planet's movement through the Zodiac. One is called Sidereal Zodiac, which uses the North star as the point of departure, and the other is the Tropical Zodiac. The latter Zodiac measures the movement from the Sun's position on the ecliptic on the first day of spring. This is called the Aries point at zero degrees. It moves backwards and marks the Vernal Equinox. The Tropical Zodiac also marks the beginning of each of our three other seasons.
Thousands of years ago the Sidereal and Tropical Zodiacs were congruous in movement. But today, due to the precession of the equinoxes, the Aries point at zero degrees in the Tropical Zodiac is today equated with six degrees in the constellation of Pisces. So, when you look at the heavens you are seeing the constellations in the Sidereal Zodiac.
Most of today's Western astrologers calculate charts using the Tropical Zodiac. This is a point of great contention between the astronomers and astrologers, in which the astronomers regard the astrologers with disdain for not using the 'real' sky to calculate planetary positions.