LPFP sits in the tank and feeds the HPFP a constant 72psi; HPFP is mounted on the engine and is chain driven. HPFP takes this input and bumps it to 700-2500ish psi depending on load. LPFP is what primes when you first start the car, that's the whirring noise that can be heard when unlocking the car.
If the HPFP dies, the car goes into limp mode but can still run off the LPFP. If the LPFP dies, the car won't run at all.
Some more in-depth reading is available here, check out the engine management PDF :
http://www.e90post.com/forums/showthread.php?t=38856