Hi, some help here:
74HC595 Serial-In-Parallel-Out Expander This IC is a Shift Register so that 8 bits of data can be 'shifted' into it over just 2 wires. This is often used to add more functional Digital Outputs to Arduino. It is most often controlled by the Arduino standard function shiftOut. See the Reference Page: http://arduino.cc/en/Reference/ShiftOut
Multiple 74HC595 chips can be connected in series so that the same 2 or 3 Arduino pins can control 8 or 16 or 24 etc. output bits.
Arduino shiftOut Tutorial: http://arduino.cc/en/Tutorial/ShiftOut
74HC595 DATA SHEET: and more available on the ArduinoInfo WIKI here - http://arduinoinfo.info
See: http://arduino-info.wikispaces.com/Popular-ICs and scroll down to the 595...