This basic wheeled robot uses very few parts, so it can built in under 5 minutes. The simple program provided shows an easy way to drive it by remote control.
The standard design includes a smooth fixed balance wheel in the back, which allows the large front wheels to both drive and turn the robot easily. Instructions are also provided for an alternative rear balance wheel which is a spinning castor wheel. This kind of balance wheel can provide less turning resistance in some cases (such as spinning tight turns on carpet), but, like the front wheels on a shopping cart, it affects the driving of the robot in other ways and can make maneuvering not as precise.
Give both a try and see which you like better!