The LSystems SOP allows for the definition of complex shapes through the use of iteration. It uses a mathematical language in which an initial string of characters is evaluated repeatedly, and the results are used to generate geometry. The result of each evaluation becomes the basis for the next iteration of geometry, giving the illusion of growth.
The example networks located in this demonstration should be enough to get you started writing custom LSystem rules.
However, anyone seriously interested in creating LSystems should obtain the book:
The Algorithmic Beauty of Plants, Przemyslaw Prusinkiewicz and Aristid Lindenmayer
For a full list of LSystem commands, see the Houdini documentation.