Though some people use their names interchangeably, griddles and grills are two different cooking products. While some foods could be prepared on either of these, some do better on one versus the other.
The best way to tell them apart is that a griddle has a flat surface, whereas a grill has raised ridges that have two functions. They keep steaks, hamburgers, chicken and other foods from sitting in the grease and similar to barbecue grills, they leave appealing grilling lines on foods as they cook. Grills can also be used for making Panini and other grilled sandwiches, with the use of a bacon press or other weighty accessory.
Common uses for griddles are French toast, pancakes, bacon and eggs, though you can cook a steak, vegetables and other foods on this cooking appliance.
Both the grill and griddle are popular cookery implements, but the fitness conscious usually prefer slenderer cooking with a grill. Either one come in models designed for use on a cooktop or as an electric appliance.
There are a few combo units on the market that give you the option of grilling or griddling with versatile interchangeable or reversible cooking plates. A combination griddle/grill can save you money as well as storage space, if you want both types of cookware or appliances. If you own a professional type range, and are unsure whether to order the manufacturers grill or griddle, it has been my personal experience that the Grill is more often used than the pancake griddle, by 80% of my customers who purchased professional range from my stores. When I had customers who could not make up their mind to buy the grill or griddle, I always recommended buying the combination unit. With a combination unit, you just turn the grill over and it is a griddle and vice versa.
Pictured below on the top is the griddle, flat bottomed, smooth surface, and below it the grill.
Note the raised edges on the grill.