A limerick is a five line poem.  The first, second, and fifth lines rhyme and so do the third and fourth.  The first, third, and fifth have the same verbal rhythm (meter) and length, and so do the second and fourth.

Examples:
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
-- Vyvyan Holland

Would a limerick thread be kinda tough?
It could work if we helped out enough.
We just need to strive through,
And do all we can do,
And maybe this thread won't be rough.
-- Dr. Monkey

The more-or-less complete rules of limericks can be found here:
http://en.wikipedia.org/wiki/Limerick_(poetry)

There once was a man named Fred
Who never would get into bed
He would not sleep
He's in too deep
And soon we found he was dead

There once was a thread with a ship
The captain we forgot to tip
The ship sunk away
But we will still play
With new places to take a trip

This great loss I will forever mourn
A burden I never thought would be born
It makes me sad
Which makes me mad
And my hair will now be very torn