A limerick is a humorous form of poetry that rhymes and has five lines. Many limericks are dirty.
When you study literature, you'll probably read some sonnets, sestinas, haiku, and plays written in verse: there might not be much attention given to limericks. That's because limericks are a more popular, non-literary form of poetry: they're mainly humorous and often sexual. A limerick has five lines and follows the rhyme scheme aabba, using what is called anapestic meter (which has to do with syllables). Many limericks involve a man from Nantucket, usually doing something unprintable with a bucket.