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.
Description
Number of Bits : 8
Pin Count : 16
Logic Family : HC
Logical Function : Shift Register
High-Current 3-State Outputs Can Drive Up to 15 LSTTL Loads
Low Power Consumption
Output Type : 3-State
Propagation Delay Time : 285ns
Operating Supply Voltage (Typ) : 2.5/3.3/5V
Operating Temp Range : -55C to 125C
Operating Supply Voltage (Min) : 2V
Operating Supply Voltage (Max) : 6V
Operating Temperature Classification : Military
Mounting : Through Hole