The difference between a griddle and a frying pan is that the frying pan is used to cook foods on the stove and has sides around it and is not a flat surface all the way around.
A griddle is sometimes also placed on a stove top but is instead a complete flat surface that is used to griddle foods such as hot dogs, sausages, pancakes etc.
Electric Griddles are just a flat cooking surface that has no sides and makes it easy to flip foods over to cook while a frying pan has sides to help keep some grease in to fry foods in the pan while the griddle is more used for grilling foods and cooking foods that do not require much if any grease.
The frying pan also allows you to use liquid ingredients you can't use when cooking on the griddle as the liquid would just roll off the griddle but not when using a frying pan.