History
Constellations are groupings of stars that, to ancient peoples, were heavenly manifestations of known or imagined objects, creatures and beings. Constellations appeared at certain times of the year, returning to those same positions annually. The ancient stargazers ascribed significance to these observations. Though identifying constellations predates the Greeks and Romans, the 48 "classical" constellation names familiar to us come from those societies.
Types
Constellations are named for the mythical beings, shapes or creatures they honored or resembled in the minds of the ancients. For example, the Orion constellation forms a man wielding a club and wearing a belt and sheath. The "Big Dipper" looks like a soup ladle. Taurus resembles a bull's head.
Features
Constellations, like the sun or moon, rise and set predictably. The earth rotates and the stars appear to circle around a central axis. Constellations help separate the myriad of stars into manageable sections. Both astronomy and astrology have names for the constellations, but while some appear in both disciplines, not all match.