An asterism is a small pattern or collection of stars that are not constellations, although they are often parts of a constellation: eg. the constellation Ursa major (Great Bear) contains the Big Dipper/The Plough asterism. Other asterisms include The Summer Triangle, The Winter Triangle, and Kemble's Cascade.
For more information about Asterisms, check out: http://www.deep-sky.co.uk/asterisms.htm
This image shows the asterism 'The Plough/The Big Dipper' within the constellation of 'Ursa Major.' You can also see light pollution on the horizon.
Comments