A grill cooks meat, fish and veggies, usually through heat from below. At the bottom of the grill is a heat source and at the top is the cooking rack. Depending on the type of grill, coal, gas or an electric element produces high temperatures, which sear the meat. A smoker grill allows the griller to add wood smoke to give a more intense, complex flavor to the food.
A water smoker is set up in a very similar way to a grill. The smoker is an enclosed cylinder with a heat source--such as a charcoal burner or electric element--at the bottom. Above that is a water pan, and above that is the cooking grill rack and chimney. The cook gets the fire going and then places wood soaked in water, juice or some other liquid on top of the fire. The wet wood doesn't burst into fire but smolders slowly while releasing a lot of smoke. That smoke rises to the meat or fish, giving it a smoky flavor as the heat from the fire cooks it. The water slowly evaporates into the air, moistening the food. Since smoking can take many hours, the food would get quite dry and tough without the moisture.
Barbecue pits are an alternative to smoker grills. The smoke flows from a firebox into a separate chamber, where the food is placed. By the time the smoke gets into this chamber, it is much cooler. Because of this, barbecue pits take much longer to smoke foods and can give them a much more intense, smoky flavor. Ribs are often smoked for as long as 24 hours before they are served. Because of their low temperatures, pits can also be used to smoke cheeses, which would melt in higher temperatures.