Chris:
SOme have pointed out that the "green stuff" is the fibre-glass epoxee material of which the printed circuit board is commonly fabricated. However I got to thinking about your question and concluded that you may instead be inquiring about the grean coating that is put on the PC board over the copper traces after the board has been etched. This coating is called solder mask and is often colored green. It can however be had in a number of other colors including I have seen blue, clear, white, purple, black, red and even yellow. Solder mask has three main purposes:

a) It aids in the fabrication process to prevent solder on pads and in holes from shorting over to other traces. This is most applicable to boards that are flow soldered and those that are screen printed with solder pasted for IR reflow.

b) It seals the board surface so that copper traces are not exposed to air and moisture which can lead to corrosion.

c) It insulates copper traces to some degree so that there are not accidental shorts when working with boards during testing.

Michael Karas