The main difference between a belt driven blower and a direct drive blower is the location of the motor. The direct drive blower has the wheel/impeller directly mounted to the motor shaft.
The belt driven fan has the wheel/Impeller mounted on a separate fan shaft and is connected to the motor by belts and sheaves. Belt driven fans are mandatory in an environment where high temperature or air contaminants would interfere with the motor. Belt driven blowers are also more adaptable to performance changes by simply changing sheave sizes.