Omni Directional Wheels can be used to move your robot in any direction. Also they can be used as castor wheels.
Omni directional wheels are special wheels with free rotating rollers around the circumference to the main wheel. The rolling direction of rollers is perpendicular to the main wheel and so the wheel can drive robot in forward-backward directions and slide on sideways. The rollers are arranged around the circumference in a way that at any point of time atleast one roller touches the ground. This wheel comes with 16 rollers- 8 on each side. Rollers coated with durable high quality rubber which helps in getting a good grip over the ground.
The ability of moving in any direction of the omniwheels allows the design of the robot which can move in any direction without taking turns or rotating. This kind of robot requires minimum 3 omni wheels. Omni wheels are also oftenly used as castors for big size robots.