"Fast food" restaurants are places where you can get cheap food quickly. In the U.S., hamburgers are the most common fast food. There are also fast food restaurants for tacos, fried chicken, and sandwiches.

You can talk about the food itself as "fast food":

I try not to eat too much fast food.

Or you can talk about the restauraunts that sell fast food:

There sure are a lot of fast food places around here!

If I'm on a road trip, I usually just stop and eat at a fast food restaurant.