To my knowledge, tank and skid steering are not the same, but they are very similar.

Tank steering is the vehicle turns about the centre of the points of contact. This doesn't matter if you use tank tracks, 4 wheels, 6 or what ever.

Skid steering the robot spins about a point that is not the centre. Typically this is done by using 4 wheels where the front 2 grip and the back two skid or slide. Razer was easy to steer as it pivoted about the centre of its front two wheels, right under the weapon tip, thanks too its back wheels being omniwheels and allowing it to slide side ways.

(I hope that makes sense)