I2C (short for Inter-Integrated Circuit) is a two-wire serial bus typically used deep inside computers for low-level communication between components. If you've ever upgraded your computer's memory, for instance, maybe you noticed the tiny extra chip on the memory sticks. This chip contains information about the speed and capacity of the RAM, and when the computer is first powered up this information is read through an I2C connection. I2C is also popular in robotics. All manner of sensors and actuators are available in an I2C-compatible form: ultrasonic rangefinders, sensors for acceleration, tilt, temperature and pressure, servo controllers, and bus expanders that provide additional general-purpose (GPIO) lines. Most modern microcontrollers have support for I2C built right in. But the processing power available on microcontrollers is limited, and software development - with specialized cross-compilers and programming environments - can sometimes be a chore. With laptops and single-board computers becoming ever smaller and more affordable, it's increasingly common to see these systems used directly in robotics and electronics projects. This provides ample power for new capabilities such as vision processing and more sophisticated A.I., and it greatly expands the scope of available development tools and languages...but it also presents a new problem: interfacing these "regular" systems to peripherals is typically done through mainstream consumer-grade ports such as USB; there's no externally-available "I2C port" we can just tap into to make use of our sensors...or is there?
http://www.instructables.com/id/Worlds-Cheapest-I2C-I-Squared-C-Adapter/