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.
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.
Here's the basic specifications: