Bouillon~ what is it and what do you do with it?

Food.com states, "Any broth made by cooking meat, poultry, fish, or vegetables in water. The liquid strained off after cooking is bouillon. A bouillon cube is a compressed, flavor-concentrated cube of dehyrated meat, poultry, fish, or vegetable stock. Bouillon granuales are the granular form of the dehydrated concentrate."

Many recipes call for "broth": chicken, beef, ham, vegetable, or fish. What do you do if you don't have any on hand? I always keep bouillon cubes and granules in my pantry. They store well and are super handy when you don't have any cans of broth in the cupboard.