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. Simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started.
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.
There is an Arduino Leonardo version without header, more suitable for hacking and finished projects.
This USB to Serial board converts a USB connection into 5 volt TX and RX that you can connect straight to the Arduino Ethernet or any other boards with an FTDI cable-compatible connector. This adapter can easily connect to the Arduino Ethernet, Mini, Mini Pro, Lilypad, LilyPad Simple, and Fio. It has also an onboard polyfuse limits the current to 500mA and protects the host computer from short circuits.