It's probably a pi network, or maybe a ladder network. Basically, it's a low-pass filter intended either to keep RF that's on the power line out of the load, or to keep the load from putting RF into the power line. Or possibly both. (The filter works in both directions, but RF of concern is probably just coming from one direction.)
Edited to add: "EMI" means "Electromagnetic Interference."
[This message has been edited by SolarPowered (edited 03-27-2006).]
