Shift registers - how to multiply the i/o lines of your microcontroller

	PINS 1-7, 15	Q0 " Q7	Output Pins
	PIN 8	GND	Ground, Vss
	PIN 9	Q7"	Serial Out
	PIN 10	MR	Master Reclear, active low
	PIN 11	SH_CP	Shift register clock pin
	PIN 12	ST_CP	Storage register clock pin (latch pin)
	PIN 13	OE	Output enable, active low
	PIN 14	DS	Serial data input
	PIN 16	Vcc	Positive supply voltage

Shifting Out & the 595 chip

At sometime or another you may run out of pins on your Arduino board and need to extend it with shift registers. This example is based on the 74HC595. The datasheet refers to the 74HC595 as an "8-bit serial-in, serial or parallel-out shift register with output latches; 3-state." In other words, you can use it to control 8 outputs at a time while only taking up a few pins on your microcontroller. You can link multiple registers together to extend your output even more. (Users may also wish to search for other driver chips with "595" or "596" in their part numbers, there are many. The STP16C596 for example will drive 16 LED's and eliminates the series resistors with built-in constant current sources.)

Read more: http://arduino.cc/en/Tutorial/ShiftOu

Shift register @ Wikipedia

In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, in which the output of each flip-flop is connected to the "data" input of the next flip-flop in the chain, resulting in a circuit that shifts by one position the "bit array" stored in it, shifting in the data present at its input and shifting out the last bit in the array, at each transition of the clock input. More generally, a shift register may be multidimensional, such that its "data in" and stage outputs are themselves bit arrays: this is implemented simply by running several shift registers of the same bit-length in parallel.
Shift registers can have both parallel and serial inputs and outputs. These are often configured as serial-in, parallel-out (SIPO) or as parallel-in, serial-out (PISO). There are also types that have both serial and parallel input and types with serial and parallel output. There are also bi-directional shift registers which allow shifting in both directions: L→R or R→L. The serial input and last output of a shift register can also be connected to create a circular shift register.

Read more: http://en.wikipedia.org/wiki/Shift_register