The 74HC595 is very useful in expanding how many pins a microcontroller can write to. Use 3 to write to 8!
The 74HC595 is a very handy IC used in many microcontroller projects. You clock in 8 bits of data (like, on/off settings for 8 LEDs) via two lines, and when you toggle a third line, it pops these settings out on 8 outputs on the IC. So you trade 3 valuable lines on your microcontroller for 8 outputs - a pretty good deal, eh?
This is called "Shifting data out" of the microcontroller by "synchronous serial communication". This is the serial part of the deal, where each bit is "shifted in" one at a time, then BOOM, they all appear at once (in parallel) on the chips output.
There are some great tutorials on how to use a shift register. Here's one for the Arduino microcontroller, but the technique holds true for pretty much all microcontrollers!
Here's the basic specifications: