Three speed modes are specified: Standard; 100kbps [Bits per Second], Fast mode; 400kbps, High speed mode 3.4Mbps. I2C, due to its two-wire nature (one clock, one data) can only communicate half-duplex. The maximum bus capacitance is 400pF, which sets the maximum number of devices on the bus and the maximum line length.
The interface uses 8 bit long bytes, MSB (Most Significant Bit) first, with each device having a unique address. Any device may be a Transmitter or Receiver, and a Master or Slave. The Slave is any device addressed by the Master. A system may have more than one Master, although only one may me active at any time.
Data and clock are sent from the Master: valid while the clock line is high. The link may have multiple Masters and Slaves on the bus, but only one Master may be active at any one time. Slaves may receive or transmit data to the master.
I²C-Bus Specification [pdf]