What is a limerick?

The rhyme scheme of a limerick is aabba; the first, second, and fifth lines usually have eight or nine syllables, and the third and fourth lines have three fewer.  They became popular during Edward Lear’s time, and he might be one of the more famous poets to use limericks.  Limericks use other elements of poetry, as well, such as internal rhyme (where two words in a row might rhyme), alliteration (where the first letter of each word is the same) or assonance (where the vowel sounds are the same). 

Limericks are usually about people, and often they begin like this -- there was an old woman in blue – with a person and some kind of description (Edward Lear liked to say where the person was from).  The next lines continue to describe some kind of woe which has befallen the person – and it ends with some kind of humorous consequence (the humor depends on the poet, of course).