Catsup is the same sauce as ketchup: the thick, sweet-sour red stuff people put on chips, burgers, and hot dogs. It is only another spelling, used more in the United States, and even there it looks a bit old-fashioned now. Most bottles you buy will say ketchup. A few older American brands, diner menus, and family recipes still write catsup, and some people will tell you the two names mean two different sauces — they do not. One sauce, two spellings. If a recipe from an American grandmother says catsup, she means the ketchup in your fridge.