Price: Smokers can be had for as little as about $50USD of for $10,000 or more. If you are not sure you want to do a lot of smoking and don't want to invest a lot of money before you find out then I recommend getting a small, vertical water smoker. These are the least expensive smokers you can buy and will let you know whether you want to invest more later or not. You can do good barbecue in these smokers and for many it is the only smoker you would ever need. However, there is a lot more to smokers than these simply units. I have recommendations of the best smokers broken down between the lower priced units and the more expensive:
Fuel: Smokers can be fueled by charcoal, hardwood, electricity, wood pellets and propane. There are advantages (and some disadvantages) to these different fuels. Charcoal and wood smokers are more traditional and typically provide a more authentic flavor to your cooking. The cheapest smokers on the market at usually charcoal, but some of the most expensive use charcoal as well. Electric smokers are the most convenient smokers but many lack authentic flavor. There are computer controlled electric smokers that allow you to set up the smoker and let it run until the food is ready (like the Cookshack Amerique). Pellet smokers (link the Traeger lil' Texas) are electrically powered but burn wood pellets to provide heat and smoke. These units can be as convenient as any electric smoker, but give you the flavor of the best charcoal/hardwood smokers. Propane smokers typically heat faster and easier than electric smokers but are still easy to use. Depending on the price you can get a smoker that gives great barbecue with very little effort. One question you want to answer is how involved do you want to be in the process. Barbecue is a long and noble tradition of people sitting by the fire making great food. So, do you want to set it and forget it or do you want to take an active part in the food you cook.
Size: The smallest smokers can produce enough food for a large family (maybe as many as 20 people). The largest smokers make enough barbecue to cater a party all day long. It is important that you consider how much barbecue you want to make before you buy. If you are only going to be smoking for the family on the weekend then a small unit will be enough. If you want to be able to smoke for the company party then you will need a lot more space. Most smokers will tell you how much food you can prepare. As a general rule of thumb you need 1 pound of meat (raw) per person. That can mean a lot of food.
Versatility: There are a number of units on the market that can smoke and grill. If you want to be able to have the best of both worlds then these are the units for you. Charcoal units, like the Big Green Egg are the most common of this kind of multipurpose smoker. While many of the lower prices units will promise this feature be aware that take a great design to be both a good smoker and a good grill. Most of the lower prices do one or the other well but not both.
Name: While you may not recognize the names stamped on many smokers I do recommend that you look into the company that produces the smoker you want to buy. There are a number of "generic" units available these days that come from brand names that are simply store brands. These units offer no future of service or support. When you find a smoker that looks good to you, put that name into your browser search engine and see what others have to say about it. I have some smoker reviews that might help you find the smoker you want.