The Data and Clock lines are both open-collector with pullup resistors to Vcc. An "open-collector" interface has two possible state: low, or high impedance. In the "low" state, a transistor pulls the line to ground level. In the "high impedance" state, the interface acts as an open circuit and doesn't drive the line low or high. Furthermore, a "pullup" resistor is connected between the bus and Vcc so the bus is pulled high if none of the devices on the bus are actively pulling it low. The exact value of this resistor isn't too important (1~10 kOhms); larger resistances result in less power consumption and smaller resistances result in a faster rise time.