The clerihew is a four-line poem with a rhyming scheme of AABB, invented by Edmund Clerihew Bentley. The first line traditionally is, or ends in, a person's name; the meter is often mangled, if not ignored altogether; and the overall intent of the poem stresses entertainment over instruction, humor over fact.

Frances Stillman in "The Poet's Manual and Rhyming Dictionary" defines it as "a humorous pseudo-biographical quatrain, rhymed as two couplets, with lines of uneven length more or less in the rhythm of prose." The name of the subject usually ends the first or, less often, the second line, and the humor of the clerihew is whimsical rather than satiric. The subject can be a real or a fictional character, as long as they are well known. 

In summary, clerihews have just a few simple rules:

1. They are four lines long.
2. The rhyme scheme is AABB.
3. The first line generally names a person, and the second line ends with something that rhymes with the name of the person.
4. Meter is uneven or ignored altogether.
5. A clerihew should be funny.

Examples from Edmund Clerihew Bentley:

The people of Spain think Cervantes
Equal to half-a-dozen Dantes;
An opinion resented most bitterly
By the people of Italy.

 

The meaning of the poet Gay
Was always as clear as day,
While that of the poet Blake
Was often practically opaque.

 

I doubt if King John
Was a sine qua non.
I could rather imagine it
Of any other Plantagenet.

 

Dante Alighieri
Seldom troubled a dairy.
He wrote the Inferno
On a bottle of Pernod.