A Juicy Lucy is a stuffed cheeseburger with cheese inside of the meat instead of on top, resulting in a melted core of cheese. Two bars in Minneapolis, Minnesota, claim to be the inventor of the burger, while other bars and restaurants have created their own interpretations of the style. The burger is prepared by putting cheese between two patties of meat, and then sealing both patties around the cheese to create a single patty with a cheese core. As the burger cooks, the cheese inside melts. This has the effect of keeping the meat near the center of the burger very juicy. It also separates the cheese from the bun, resulting in a slightly different texture than the usual cheeseburger. Burger toppings such as condiments, onions, and pickles may be added.

Source: Wikipedia