The motion model of this robot is defined Differential Steering or Differential Drive or (more academically) Unicycle Robot.
Basically is a two wheels robot with a third point, the freely pivoting castor, used to balance it.
Modifying the difference of the speed between the wheels the robot can go straight, forward or backward, turn in any direction with any desired angle and even spin on a fixed position, giving to it plenty of mobility. Less efficient at high speeds but more agile on maneuver than the traditional car steering.
Click and navigate
If you want to know more about the theory of the differential steering and the equations used to calculate the position, the angle and the speed, take a look at the link to the left.
The video shows the result of this theory applied to remotely control the robot Rino.
update on 29-12-2015
Wheels

Motion

The theory behind