Difference between Quiescent Current & Shutdown current?
"Quiescent current" is the nominal (average) current used while "on" in a minimal state, such as an amplifier IC which is on and ready to work, but not currently amplifying anything. This is the minimum average current the device will consume while "on" and can be used for a minimum power consumption (maximum battery life) estimation.

"Shutdown Current" is the current used while the shutdown pin is asserted. Many IC's have a shutdown feature, and asserting this puts it into a low-power "sleep" state. The advantage to having a "shutdown" feature, usually is that it can be turned on and off with one logic signal, "plays nice" with other devices that may still be on, and wakes up quickly. More advanced IC's can allow certain features to remain running while in "shutdown", such as a PIC microcontroller and some of it's peripherals (real-time-clock-calendar, low-speed clock, interrupts, etc.) Shutdown is primarily used to save power and extend battery life.