Beef broth is made by boiling meet in water and then reserving the flavored water after the meat is removed. Beef stock is made by boiling bones in water.
While the finished products are similar, stock tends to have a slightly heartier taste than broth. While trained chefs and discriminating foodies can detect enough of the subtleties in the differences between their tastes, most people find that broth and stock can have similar tastes. For this reason, it is fine to substitute broth for stock or vice versa in most everyday recipes. Though, it is a good idea to keep both in the pantry or refrigerator for cooking.