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Space Mapping
As modern optics became increasingly available and celestial discovery flourished in the early 20th century, it became necessary for star gazers to have a map of the night sky, which they could reference when directing others to their discovery. In response, the International Astronomical Union carved the night sky into 88 constellation areas, most with their original Greek nomenclature, that form a map of visual space. Asterims, on the other hand, are recognizable patterns that sometimes cross these boundaries.
Classification
Just as a single astronomer, Eugène Delporte, is widely credited with developing the map of the 88 constellations used in modern astronomy, a single Greek instructor, Claudius Ptolemy, is credited with categorizing 48 of the constellations now recognized by the International Astronomical Union. Asterisms are recognizable patterns that fit more closely with Ptolemy's descriptions than the modern map.
Common Asterisms
Contrary to popular references, many patterns visible in the night sky aren't constellations at all. Even those that have roots in ancient astronomy are asterisms, rather than constellations. Orion's Belt, for instance, is actually an asterism within the constellation of Orion. The Big Dipper, perhaps the most recognizable star pattern in the northern hemisphere, is also an asterism, consisting of the brightest stars in the Ursa Major (Great Bear) constellation.
Interpretation
In many ways, there is little or no difference between a constellation and asterism. Most ancient cultures had some method and nomenclature for recorded star gazing. Evidence from ancient Chinese, Babylonian, Egyptian and Greek societies suggests that scholars in these societies recorded patterns in the night sky. Some even included distant star clusters that appear as light clouds to the naked eye. Under modern classification, all of these patterns would be considered asterisms.