DIY LED Array Display - using ATmega168 microcontroller

NerdKits has put together a video that describes how a LED row/column matrix is wired. This comes in handy when you have limited pins and need to control lots of items. Another popular method is to use a serial to parallel latch.

"This project makes use of row-column addressing, which lets us control lots of LEDs without having one wire going to each one. We also use one more trick — the fact that diodes only conduct in one direction — to double the number of LEDs you can handle with the ATmega168 microcontroller. "

From - Hackedgadgets