Michael has nailed it. But, I also have to say that braising in barely simmering liquid, though it does a wonderful job, isn't the only way to cook a great stew. As ahs been stated by two former posters, you can achieve wonderful results by using a pressure cooker set at 15 lbs. This reduces cooking time dramatically, even though the liquid boils inside the cooker. For whatever reason, it comes out moist and tender. Also, when a pressure cooker is used, flavorings tend to permeate both the meat and veggies, i.e. potatoes, carrots, celery, onions, etc. The down side is that you have to make sure to use enough water for the cooking time as it is somewhat difficult to remove the lid to add more. You have to cool everything down and release the pressure. Then, you have to heat it all up again. But as a general rule, if you fill the pot with enough water to cover the food contents, you will be fine. Cook for about 45 minutes to get a great stew.
If you don't have a pressure cooker, then use a slow cooker, or slow oven and a heavy, lidded pt, like an oval roasting pan, or a dutch oven.
Seeeeeeya; Goodweed of the North