The I2C (Inter IC) bus was developed by Philips in the early '80s, for use in communication of peripheral devices within a TV-set. In some implementations, I2C is called as TWI (Two-Wire Interface) due to royalty issues. I2C is actually pronounced as "i squared c" (I2C), but most pronounce it as "i two c".
From its name we can easily deduce that it provides a communication link between ICs (integrated circuits). I2C is multimaster and can support a maximum of 112 devices on the bus. The specification declares that 128 devices can be connected to the I2C bus, but it also defines 16 reserved addresses.
Devices that support I2C can be directly connected to the bus. Analog devices can be connected to the I2C bus by using ADCs (ie Philips PCF8591, Analog Devices AD7992) or I/O expanders (ie TI PCF8574, PCF8575) with an I2C interface.
Communication speeds of 100kbs (kilobits per sec) in Standard Mode or 400kbs in Fast Mode should be attainable. A new revision of the specification allows for a maximum throughput of 3.4Mbps (megabits per sec) in High-speed Mode. Some documents will state these as 100kHz, 400kHz and 3.4MHz respectively, they are the same.