A cafeteria is a restaurant where you serve yourself, then pay a cashier. Most schools have a cafeteria serving up sloppy joes and square pizza.
"Let's go to the cafeteria!" usually means "Let's eat!" Most people think of school cafeterias, but they can exist elsewhere — a lot of Ikea stores actually have cafeterias for the customers. Cafeteria-style refers to any restaurant where you grab what you want and then pay for it before eating. Cafeterias usually aren't known for having the best food in the world, so if you have another option, go for it.