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 into a range. Electric griddles (here's one) are also available.
Griddles are also often used for cooking burgers and other hot sandwiches.
A commercial griddle (i.e. a griddle that is built into a range top) 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.