The constellations are totally imaginary patterns that poets, farmers and astronomers have made up over the past 6,000 years.  The real purpose for the constellations is to help us tell which stars are which.  On a really dark night, you can see about 1000 to 1500 stars. It's hard to tell which is which.  The constellations help by breaking up the sky into more manageable bits. They are used as mnemonics, or memory aids.  In all there are 88 constellations which have been named.  About half of them can be seen on any clear night.  The signs of the zodiac are 12 constellations, which make a ring around the sky, and the planets are always somewhere in this ring. 

Since different constellations are visible at different times of the year, you can use them to tell what month it is. For example, Scorpios is only visible in the northern hemisphere's evening sky in the summer. Some historians suspect that many of the myths associated with the constellations were invented to help the farmers remember them. When they saw certain constellations, they would know it was time to begin the planting or the harvesting.