What is the difference between a constellation and an “asterism”? name at least one commonly known asterism.

A "constellation" is the total, full, complete thing in the night sky ...
ALL of the stars that go together to make up the complete dog or
the queen or the fish or the bear, or whatever else the story of
that particular part of the sky involves.

An "asterism" is a small, easily recognized group of stars, that
can be spotted right away, although they're only a small part
of a constellation.

The Big Dipper, the Little Dipper, Orion's Belt, and the Great Square
are asterisms.  They're parts of the constellations of the Big Bear,
the Little Bear, Orion the Great and Mighty Hunter, and Pegasus,
respectively.