the old fogies of the Arduino world, the Uno and Mega, actually have two chips. An ATMega16u2 takes care of the USB connection, while the standard ‘328 or ‘2560 takes care of all ~duino tasks.

It’s a significant update. The Arduino UNO and Arduino Mega 2560 update the Duemilanove and Mega respectively. They now use an ATMega8U2 instead of an FTDI USB to serial converter chip.

If you're still confused about which to pick, here are five use cases when the Arduino microcontroller board ... set of 76 pins and the Arduino Mega 2560 with a total of 70 pins.