There was a young girl from Uttoxeter…

Limericks are a type of light verse, usually consisting of five anapaestic lines rhyming aabba, the a-lines being trimeters, the b-lines dimeters.

A better answer is that a limerick is a poem like this:

The limerick packs laughs anatomical
Into space that is quite economical,
But the good ones I’ve seen
So seldom are clean,
And the clean ones so seldom are comical.

You can’t write a proper limerick about Limerick. Turmeric? Gymrek? So Limerick folks can dish it out to all those capacious actresses from Crewe, asymmetrical men from Devizes and unspeakably naughty Bishops of Birmingham, knowing that none of them can get his own back. It could well be, incidentally, that there are no more limericks in Limerick than there are nuns in Covent Garden or shepherds – or bushes – in Shepherd’s Bush. (Pratts Bottom is an exception on both counts.)

Limericks were popularised by Edward Lear, although his were not called limericks at the time. They were dull:

There was an old person of Dean
Who dined on one pea, and one bean;
 For he said, “More than that
 Would make me too fat,”
That cautious old person of Dean.

I realise that the last line is not meant to be a punch line, and that the repetition is deliberate, but it is still dull. As is this one:

There was an Old Man in a tree,
Who was horribly bored by a Bee;
When they said, ‘Does it buzz?’
He replied, ‘Yes, it does!’
‘It’s a regular brute of a Bee!’

The latter was parodied in “A Nonsense Rhyme in Blank Verse”, attributed to W. S. Gilbert: 

There was an old man of St. Bees,
Who was stung in the arm by a wasp;
When they asked, “Does it hurt?”
He replied, “No, it does n’t,
But I thought all the while ‘t was a Hornet!”

The rigid rules of meter, length and rhyme make the limerick form easy to subvert.

Rhythm isn’t everything, as that actress said to that Bishop, somewhere between Crewe and Birmingham:

There was a young man of Japan
Whose verses never would scan.
When they said, “But the thing
Doesn’t go with a swing,
He replied, “No, but I try to get as many words into the last line as I possibly can!”

“Nor is length,” squealed the girl from Uttoxeter, “At least not from where I’m sitting.”

There once was a man from Peru
Whose limericks stopped at line two.

And as for rhyme, for avoidance of smutty innuendo, it is best to pull out at the last minute:

There was a young lady from Bude
Who went for a swim in the lake
A man in a punt
Stuck an oar in her ear
And said “You can’t swim here, it’s private.”

It is fun to break the rules, but my two favourite limericks obey them. One has already seen the light of day in a blog post, but it deserves another outing: