Outdoor grills are almost always gas powered, while most indoor grills are electric. Indoor grills can be open, like outdoor grills, but some are folding models. These folding models allow for efficient grilling of sandwiches, much like a panini press.
Some outdoor grills are charcoal fired. This can cause a major difference in flavoring of grilled foods. An indoor electric grill will not cook food to the same flavor as an outdoor gas grill, while some vented, gas-powered indoor grills come close. Despite this difference in flavor, the supposed health benefits of electric grills are often exaggerated, according to About. Indoor grills burn fats to the same degree as outdoor grills, and thus there is no difference in the amount of carcinogens produced by the grilling process. Indoor grills are touted for their fat-reducing properties, but there is no difference in the fat contents of a pan fried and an electric fried hamburger, according to About.
Another major difference between indoor and outdoor grills is that many indoor models are able to fold. This allows for simultaneous grilling of both sides of the food and doesn't require flipping. These models are commonly used for grilling sandwiches and require less skill of the cook.