Flavorful stocks are always made from the bones of animals because of the collagen they contain. Through cooking, collagen is converted to gelatin and water, giving a stock it's texture and richness.
The difference between a stock and a broth is that you make a stock from bones, you buy a broth at the grocery store.
There is so much more to stock making than simply the amount of time you spend simmering the bones. Firstly, never BOIL a stock, always a soft simmer. Boiling will cause many of the impurities to be suspended in the liquid, giving you a cloudy stock.
Always start stock with cold water. Cold water will allow the impurities to congeel and can be skimmed from the top of the stock during cooking. Hot water will dissolve the impurities, giving a cloudy stock.
Stocks should be strained and chilled as quickly as possible. After running your finished stock through a cheese cloth, it should go in a container that's set in an ice-bath. The quicker you cool the stock, the more fat will float to the top to be skimmed away.
If you're making a white stock, like chicken, 3-4 hours is certainly sufficient. If you're making a brown stock, the bones and flavoring agents should be browned in the oven first to caramelize sugars and give that brown color.
Stock making is a delicate process and has a lot more to it than just cooking times.