"A light, humorous style of fixed form poetry. Its usual form consists of five lines with the rhyme scheme aabba; lines 1, 2, and 5 contain three feet, while lines 3 and 4 usually contain two feet. Limericks range in subject matter from the silly to the obscene..."

Definition by: Glossary of Literary Terms

A: What is a limerick, Mother?
A: It's a form of verse, said brother
B: In which lines one and two
B: Rhyme with five when it's through
A: And three and four rhyme with each other.