Most RC servos are designed to rotate 180 degrees – 90 degrees clockwise and 90 degrees counter clockwise. The speed and direction of rotation is controlled by a PWM signal – “pulse width modulation”.
Since a servo is basically a motor which has a gear-reduction, it is very close to what is required as a drive motor for a small toy or robot. The trick is to eliminate the restriction on how far it rotates. A continuous rotation servo rotates indefinitely in either direction and the PWM controls how fast the motor turns.
There are many videos and tutorials on the internet – search for: ‘convert servo to continuous’ – and each servo manufacturer will need slightly different instructions.
TBH: If you can find a continuous rotation servo for less than about $5 over the cost of a regular servo, it’s worth the money. Converting a single servo will usually take 15-20 minutes and require two 2.2K ohm 1% resistors.