A bipolar stepper motor has one winding per stator phase. A two phase bipolar stepper motor will have 4 leads. In a bipolar stepper we don`t have a common lead like in a uni-polar stepper motor. Hence, there is no natural reversal of current direction through the winding.
A bipolar stepper motor has easy wiring arrangement but its operation is little complex. In order to drive a bipolar stepper, we need a driver IC with an internal H bridge circuit. This is because, in order to reverse the polarity of stator poles, the current needs to be reversed. This can only be done through a H bridge.