Broth and Stock are two kind of liquid food preparation.
Stock is traditionally prepared by simmering various ingredients in water. It can be made just with vegetables (usually a mirepoix). Or with addition of herbs and spices, meat/bones, or fish. Solid parts are then removed, leaving just the flavored liquid part.
Broth can be made starting from simple water, just like stock, or from an already flavored stock, simmered with more ingredients, like meat/bones, or fish, herbs and spices, sometimes cereals/grains, and, of course, vegetables too.
Both can be used as a basis to make other dishes, like soups and sauces, or even risottos. You can also use them to cook some food and make stew.