Asterism refers to a pattern of stars as seen from the Earth, but which is not a constellation. It is made of stars that are not related physically and may be of great distances from each other, but are visible when looking from a particular direction.
Unlike constellations, asterisms can be made up of varying, unspecific stars. They can even be composed of stars that are marked as members of certain constellations. In addition, most asterisms show simple shapes and few stars, making them easy to spot in the night sky. Hence, they are quite helpful who are just getting acquainted with star gazing and mapping.
To children and to those who are not really into hardcore study of the sky, asterisms are equal to connect-the-dots figures. The stars in the star cluster are not related gravitationally, because if they are, the cluster is not an asterism anymore.
It is also interesting to know that asterisms are used to determine the seasons. The Diamond of Virgo marks Spring, while the Great Square of Pegasus indicates Autumn. The Summer Triangle of Deneb, Altair and Vega signals the season from which it got its name – pretty much the case for the Winter Hexagon of Sirius, Procyon, Pollux, and Castor.