A clerihew is a short poetic form about a famous person. Clerihews rhyme and consist of four lines.
Clerihew was the middle name of the man who invented the form. Like the haiku, the clerihew is a very short type of poetry with a specific form. A clerihew must have four lines and consist of rhyming couplets. A clerihew should also be about a famous person. If this all sounds serious, it isn't. Like a limerick, a clerihew is usually humorous (though not necessarily dirty, as limericks tend to be). People write clerihews for fun and to amuse.