The Zodiac is the path that the planets follow as they move relative to the background of fixed stars. You can visualize the Zodiac as a belt in the sky, about 18 degrees of arc in width, running around the earth in an east-to-west direction. Several groups of fixed stars are studded along this imaginary belt. The fixed stars are divided into two sets, one of twelve groups and another of twenty-seven groups. The twelve groups, based on the motion of the Sun are called Signs or Rāśis; the twenty-seven groups of stars are called Nakṣatras, stellar mansions or asterisms. This imaginary belt, with 12 Rāśis and 27 Nakṣatras ranged along on it, is called the Zodiac.
The heavenly bodies called planets or Grahas move, generally from west to east, in front of the background of the fixed Rāśis and Nakṣatras. The name Graha (graha = Sanskrit to catch hold of) derives from the fact that while moving against the background of the Nakṣatras, they appear to get hold of one Nakṣatra after the other. Vedic astrology recognizes nine Grahas: Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, Rahu and Ketu. Of course, the Sun is a star, the Moon is a satellite of the earth, and Rahu and Ketu are mathematical points on the Zodiac, but Vedic astronomy and astrology refer to all of them as Grahas. The Grahas (appear to) revolve around the earth along the path of the Zodiac.