Comfort food is usually stemmed from something that you can remember eating at a specific time that made you feel good, and I'm not talking about having a good donut in a shop or something. I'm talking about grandma's stew, your mom's spaghetti, your uncle's "famous BBQ chicken," any food you remember having at a family or friend gathering and brings back good memories or foods that your grandparents and parents made when you were a kid and food you grew up with.
This varies greatly from person to person, but generally speaking, foods like macaroni and cheese, mashed potatoes, meatloaf, thick soups and stews, and fresh baked bread would all be considered comfort foods. Hot, high in fat, and loaded with flavor.
Now I, growing up in a very Hawaiian household, have a different take on comfort foods. Poi, dry fish, plate lunch, crackseed, and Vienna sausage and eggs are MY comfort foods. They remind me of my "small kid time" with my grandparents and make me feel good :)