A stepper motor steps. Hence, the name.
If the steps are performed in one order (A, B, C, D, ...), the motor will rotate one way. If the steps are performed in the opposite order (..., D, C, B, A) the motor will rotate the other way.
Typically, a motor driver board will take care of making sure that the motor steps in the forward or reverse directions for you.
Driving all but the tiniest stepper motors directly from the Arduino (or Teensy) is not advised.