This is simply the ATmega 328P microcontroller, used on the Arduino UNO development boards. In order to use it in a circuit, you'll need to connect an external crystal oscillator and decoupling caps and provide power to various pins. You can use the schematic of the Uno to make sure you connect it up properly.
Please note that this microcontroller does NOT have the Arduino bootloader pre-installed. It can however be relatively easily installed using the instructions on this page.