National Endowment for the Humanities
Neh: Edsit Ement: There Once Was...
Part 2 of this tutorial on limerick writing demonstrates the importance of form (meter and rhyme) in limerick poetry using the exemplary and amusing work of Edward Lear.
Emory University
Emory University: The Limerick
This site gives a good description of limerick. Features details about the rhyme scheme and the different forms of limericks. References to the bawdy nature of this verse form.