A limerick is a humorous verse of three long and two short lines. It's rhyme scheme is (AABBA). A limerick was popularized by Edward Lear, but he did not use limerick to describe his poetry.
Example of a Limerick
By: Edward Lear
There was a Young Lady whose chin
Resembled the point of a pin:
So she had it made sharp,
And purchased a harp,
And played several tunes with her chin.