A French top looks similar to a griddle: It's a large, flat surface typically made from stainless steel that sits on the rangetop. Unlike a griddle, a burner is placed below the center of a French top. Marked by a circular, removable plate, the center of a French top is the hottest point. The further away you get from the center, the more the surface temperature diminishes. Pots and pans can be placed directly on the French top, or the plate can be removed to place a pan directly over the burner, or to put a wok ring over the burner.
French tops are typically found in commercial settings, and are usually only offered by high-end residential range manufacturers.
Why you'd want one: A French top can be beneficial if much of your cooking revolves around slow cooking and cooking sauces. French tops allow you to easily maneuver cookware from hot areas to cooler areas without having to change the burner setting or rearrange cookware from one burner to another.