C bus, or Inter-Integrated Circuit
Bus, was developed by Philips
around 1980, as a way to connect the various ICs ("chips") inside a television set.
Before I2C and similar busses, individual ICs in the TV would have to have dedicated control signals from the TV's microprocessor or microcontroller. One IC would control the tuner, one would control the audio circuitry, one might control the focus and other such parameters.
I2C replaced all these dedicated control signals with a serial bus using just a pair of signals. The I2C has a Serial CLock signal (SCL) and a Serial DAta signal (SDA). The microcontroller uses a standard protocol to individually addres one of the devices on the bus, and then set control parameters and read back status values. I2C devices are now found in a great deal of electronic products aside from televisions.
The System Management Bus (SMBus), found on newer motherboards for things like temperature sensing, is compatible with the I2C bus.
Note that Philips has a patent on I2C, and demands license fees from manufacturers of I2C slave devices. Other serial busses, such as SPI and Microwire do not have such issues.