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Constellations are groupings of stars that form easily recognized and remembered patterns. Actually, the stars in the majority of all constellations do not 'belong together'. Usually they are at greatly varying distances from Earth and just happen to lie more or less in the same line of sight as seen from our solar system.

Although the constellations are due mainly to the optical accident of line of sight and have no real significance, astronomers have retained them as reference areas.

Egyptian god of the underworld

If you would colour the circle formed by the yearly passage of the earth around the sun yellow, and you then make this circle a lot bigger, you would find that all the other planets (except Pluto, who is no planet any longer, poor Pluto) are lying on this same frisbee. All the planets circle the sun on this same plane, called the plane of the ecliptic. Because of the earth's yearly revolution around the sun, the sun appears to move in an annual journey through the heavens with the ecliptic as its path. 

The constellations through which the ecliptic passes are the constellations of the zodiac, the zone of the sky that includes about 8° on either side of the ecliptic.