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)