I2C (also referred as IIC or TWI) is widely used interface in embedded applications. Two wire bus initially was used by Philips and become a standard among chip vendors. I2C bus consists of two lines called Serial Data Line (SDA) and Serial Clock Line (SCL). Communication is relatively fast and short distance mainly used to communicate between sensors, RTC, EEPROM, LCD. I2C protocol allows up to 128 devices connected to those two lines where each of them has unique address. Communication between devices is master and slave based. Master generates clock signal, initiates and terminates data transfer.
Programming AVR I2C interface - [Link]