A griddle is a piece of cooking equipment where the cooking surface is flat with a heat source underneath. A griddle can be a piece of cookware, such as a cast iron griddle (like this one) that is heated over a burner, or it can be built in to a range. Electric griddles (here's one) are also available.
Food is cooked directly on griddle. A griddle is good for cooking many types of food, such as eggs, bacon, pancakes, French toast, hash browns and other breakfast items. Griddles are also often used for cooking burgers and other hot sandwiches.
A commercial griddles (i.e. a griddle that is built into a rangetop) is similar to a flat top but they're different in certain respects. For one thing, those kinds of griddles tend to be made of lighter steel, and the heat source underneath the griddle is a straight heating element, not multiple round ones. Also, food is cooked directly on the griddle, whereas a flat top can accommodate cookware as well as food.