Arduino leonardo R3:

The Leonardo is Arduino?

first development board to use one microcontroller with built-in USB. Using the ATmega32U4 as its sole microcontroller allows it to be cheaper and simpler. Also, because the 32U4 is handling the USB directly, code libraries are available which allow the board to emulate a computer keyboard, mouse, and more using the USB-HID protocol!

The Leonardo differs from all preceding boards in that the?ATmega32u4?has built-in USB communication, eliminating the need for a secondary processor. This allows the Leonardo to appear to a connected computer as a mouse and keyboard, in addition to a virtual (CDC) serial / COM port. It also has other implications for the behavior of the board.
The Arduino Leonardo is a microcontroller board based on the?ATmega32u4?(datasheet). It has 20 digital input/output pins (of which 7 can be used as PWM outputs and 12 as analog inputs), a 16?MHz?crystal oscillator, a micro USB connection, a power jack, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.

Summary:

Microcontroller ATmega32u4
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limits) 6-20V Digital I/O Pins 20
PWM Channels 7
Analog Input Channels 12
DC Current per I/O Pin 40 mA
DC Current for 3.3V Pin 50 mA
Flash Memory 32 KB (ATmega32u4) of which 4 KB used by bootloader
SRAM 2.5 KB (ATmega32u4)
EEPROM 1 KB (ATmega32u4)
Clock Speed: 16MHz

Features:

ATmega32u4 Microcontroller
Input Voltage: 7-12V
20 Digital I/O Pins
7 PWM Channels
12 ADCs
16MHz Clock Speed
32 KB Flash Memory