To most people, a constellation is a group of stars which appear to form a picture in the sky, assuming one uses some poetic license. To astronomers, however, a constellation is a specific region of the sky, with astronomers breaking the visible sky up into 88 different constellations. Whether you are a layperson or an astronomer, constellations are a way of viewing the night sky and breaking the information in the sky up into usable chunks, rather than trying to take in the sky as a whole.
In the lay sense, constellations have been an important part of many cultures. People have historically singled out clusters of stars and imagined patterns which surround them, from lions to ships, and many cultures have complex myths about the constellations and their origins. Some cultures also have stories about “dark constellations,” areas of the night sky which are totally darkened. Astronomers refer to constellations in the lay sense as “asterisms,” with some people preferring “classical constellations.”
These classical constellations were adopted by astronomers when they started breaking the sky up into constellations. The constellation Orion, for example, lies within the rectangle drawn by astronomers which has also been named Orion, after the closely associated layperson's constellation. This overlap can make things a little bit confusing for people.
Many asterisms go by several names, and are associated with several different myths. Ursa Major, for example, goes by the Big Dipper in North America, and the Plough in England. Constellations often appear in creation myths, with a variety of explanations for their presence, from myths about people trapped in the sky in the form of constellations to legends about spilled jewels. The diversity of myths which involve the night sky seems to suggest that people have long been fascinated by the stars and the sky.
Although the cluster of stars in a constellation might look very close to each other from Earth, they are actually separated by considerable distances, and there are numerous stars between them which are too faint or too distant to see. In many cases, the light from distant stars simply has not reached us yet, because those stars are too new. Conversely, some of the stars which are familiar to us from their place in asterisms may not exist any more, but it may take millions of years for us to find out, since the light from the star is traveling across such an incredible distance.