Here the Arduino UNO schematic diagram (click to enlarge):
About Arduino UNO:
The Arduino Uno is really a microcontroller board based on the ATmega328. It has 14 digital input/output pins (of which 6 may be employed as PWM outputs), 6 analog inputs, a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, along with a reset button. It contains every little thing needed to support the microcontroller; just connect it to a laptop or computer having a USB cable or power it with a AC-to-DC adapter or battery to obtain began.
The UNO differs from all preceding boards in that it does not use the FTDI USB-to-serial driver chip. Instead, it functions the Atmega8U2 programmed as a USB-to-serial converter.
“UNO” means one in Italian and is named to mark the upcoming release of Arduino 1.0. The Uno and version 1.0 is going to be the reference versions of Arduno, moving forward. The Uno will be the latest in a series of USB Arduino boards, along with the reference model for the Arduino platform; for a comparison with previous versions, see the index of Arduino boards.