There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar.
4 Wires: Bipolar only
5 Wires: Unipolar only
6 Wires: Universal
8 Wires: Universal
Unipolar and bipolar is the type of connection each motor.The only difference is that bipolar can not be connected as unipolar (because it has only 4 wires) ,but unipolar can be connected ae bipolar.
Bipolar stepper motors
Bipolar motors have a single winding per phase. The current in a winding needs to be reversed in order to reverse a magnetic pole, so the driving circuit must be more complicated, typically with an H-bridge arrangement (however there are several off the shelf driver chips available to make this a simple affair). There are two leads per phase, none are common.
Because windings are better utilized, they are more powerful than a unipolar motor of the same weight. This is due to the physical space occupied by the windings.
Unipolar stepper motors
A unipolar stepper motor has two windings per phase and of course two phases, one winding for each direction of magnetic field. Since in this arrangement a magnetic pole can be reversed without switching the direction of current, the commutation circuit can be made very simple (eg. a single transistor) for each winding.