Why are constellations used?
Constellations were drawn up by the ancients, and their use was continued for astrology. Later on, the arrangements were used to provide a "structure" to the sky, even for the modern astronomer. We locate stars and other objects by the constellation they are in.
Constellations are important because our current calendars are based on the zodiacal constellations - the constellations along the ecliptic. The different zodiacal constellations were used as a tool for our ancestors to know when animals hibernate, and migrate - they used them to tell when the rainy season was coming - they used them to know when to harvest and plant their crops. Everything about our current way of life is based on watching the…
Constellations are of mainly historical importance, as a record of the way in which the human race used to think about the stars. Modern astronomers do not categorize stars according to constellations, since they have more meaningful criteria. Astrologers, of course, continue to attribute vast mystical significance to constellations.
There are 88 constellations nowadays. And by definition, a constellation isn't only the stars you're used to see in connected lines - it is a region of the sky with perpendicular boundaries including stars, nebulae, objects, etc. For a list of constellations look at http://www.astro.wisc.edu/~dolan/constellations/constellation_list.html