Fast food refers to high in fat content ready-to-serve food that you get from the stores of Burger King, KFC, McDonald's, Chick-fil-A, In-N-Out Burger etc. It is popular among busy people. Fast foods are notorious for being rich in trans fat (though some restaurants are beginning to stop using it) content and spreading obesity and type 2 diabetes. There has also been reported a higher risk in stroke occurrences with regular fast food customers, but this could also be due to other factors, such as lack of exercise.
Of course, it is still possible to occasionally enjoy fast food and remain quite healthy, as long as you also have healthier options in your regular diet and exercise regularly. Contrary to popular belief, not all people who enjoy fast food are the same. There are those who consume fast food on an almost daily basis, exercise rarely, consume a lot of other junk food, and eat little to no fruit or greens, but there are those who consume fast food in moderate to low quantities (not more than a meal or two per week) and in small proportions. While a lot of obesity can be linked to fast food consumption, having an occasional fast food meal is probably, by itself, not going to make you fat.
 Comparison with "slow food" restaurants
While it is true that the menu items at fast food restaurants tend to be very calorie-dense and fattening, the same can be said for many of the menu items found at more upscale restaurants. The Macaroni and Cheese entree at Red Robin, for example, contains over 40 grams of saturated fat (double the full daily value for someone on a 2000 calorie-a-day diet). The nutrition info for some of the menu items at the Cheesecake Factory, or Marie Callender's, would put a McDonald's Big Mac to shame.
Thus, the main health criticism of a fast food establishment shouldn't be that they have fattening menu items — as non-fast-food restaurants frequently do also — but that:
- The low cost and high-speed of delivery of their menu items makes access to calorie-rich foods more convenient than it would be at a conventional restaurant.
- The selection of lower-calorie menu items tends to be very limited.
And even "junk food" has its defenders.
 See also