All communication in I2C is done in 9-bit segments. A byte is sent from a host to a client or vice versa, followed by an Acknowledge sequence sent back. After the eighth falling edge of the SCL line, the device outputting data on the SDA changes that pin to an input and reads the Acknowledge value on the next clock pulse.
The clock signal, SCL, is provided by the host. Data is valid to change while the SCL signal is low, and sampled on the rising edge of the clock. Changes on the SDA line while the SCL line is high define special conditions on the bus, such as a Start or Stop condition.