It is a silly poem with five lines.

It is a 14-line poem written in iambic pentameter.

There once was a reader of poetry --To whom limericks seemed like magicianry -- So they turned to this sample -- Of limerick examples -- And learned a few things they’d not known, see?

It is a poem that tells a story in four-line stanzas called quatrain.

It provides the poet with space to express his personal feelings he cannot show otherwise in prose or any other form.

It is a kind of poetry very closely related to ballads - so closely, in fact, that we might say epic poem are a kind of ballad.