Polarized plugs have two different sized slots, the neutral is the larger slot, this allows for a device to be plugged in, in only one way to prevent plugging in your device incorrectly.
A three-prong design has a grounded bottom prong as an extra safety precaution. This prevents you from being shocked when plugging in your appliance.
What’s really important about this distinction between these types of outlet plugs is to make certain that the timer you purchase has the correct corresponding outlet for the device you want to control. Extension cords and household appliances typically have the three-prong grounded plugs, but we recommend checking what type of outlet you need before purchase.