Basic bone broth: put bones of some kind of animal in a stock pot, fill with water, keep on low heat for extended period of time. Add a little apple cider vinegar to break the bones down a bit.
You can add veggies to make it a stock, but all the solids get filtered out after (with cheese cloth). Some people roast the bones first for flavor, especially if you're using something with tasty marrow (like large beef bones). You can keep it really simple, or basically cook a soup for a long period of time, until all the nutrients from the solids have been extracted into the broth. Seasoning is optional, but I always add salt. Saving pan drippings is also a great addition to a broth stock, which is basically what Belforte was talking about.
With a pressure cooker: Quick Pressure Cooker Bone Broth | Award-Winning Paleo Recipes | Nom Nom Paleo