A constellation is a group of stars that can be joined together to form a shape or picture. Many ancient cultures around the world often created stories about the shapes they could see in the night sky.
Some well-known constellations contain familiar patterns of bright stars, such as Orion, which the ancient Greeks saw as a hunter, and Leo, which traces the outline of a lion.
These days, astronomers divide the sky into 88 constellations with exact boundaries (see image right), so that every point in the sky belongs within one of those constellations. We also have asterisms which are patterns of stars which are often smaller parts of a constellation.
In the northern hemisphere, these are mostly based upon the constellations recorded by the ancient Greek culture, that have been passed down through the Middle Ages.
Of the modern constellations, there are 12 through which the Sun travels over the course of the year. These are known as the "signs of the zodiac" and are thought by many to have special significance.
For more information about the 88 constellations, try this LINK.