When deciding what type of canning equipment and process to use, the first thing to know is that you should ONLY use water or pressure canning techniques! There are other older, traditional practices, but these should be avoided as they involve steps that can expose the food to non-sterile conditions during the packing, and the foods do not reach and maintain high enough temperatures to ensure safety. In fact, the USDA recommends against using any canning recipes from earlier than 1990 as they often contain directions now considered unsafe. Your primary goal in canning your foods is preserving the foods in a safe and edible condition, avoiding exposure to organisms that can seriously sicken you or others.
So, your choice is between a water canner and a pressure canner. Which to choose?
Water canning involves immersing the filled, closed jars in a bath of boiling water. Water canning is faster than pressure canning, and uses less complicated equipment. However, the maximum temperature reached in the jars is 212 degrees Fahrenheit. This is a high enough temperature to kill many organisms that can spoil food, but is not high enough to kill the bacteria that causes botulism, a serious and potentially fatal food-borne illness. However, the botulism bacteria cannot survive in highly acidic foods such as most fruits, pickles, and tomatoes (note that you need to add acid to the tomatoes, in the form of vinegar or citrus juice, to ensure high enough acidity). These can safely be canned in a water canner.
Pressure canning involves more complicated equipment and techniques, and usually takes longer than water canning. In pressure canning, the sealed canner combines with the heat and steam produced by the boiling water to raise the maximum temperature of the items beyond the 212 degree Fahrenheit boiling point of water. This heats your food above the temperature required to kill even the hardier microorganisms that can survive the water canning process. Pressure canners also generally cost more than water canners, and you have to be careful to adjust your process to account for higher altitudes, and take steps to make sure any gauges are accurate so that you are not accidentally lowering the canning temperature. But you can safely can a much wider variety of food items, including low-acidity vegetables like green beans, asparagus, and peppers; low-acidity fruits like figs; and some meat, fish and dairy products.
If you are just starting out in canning or want to stick to the simplest items such as jellies and pickles, a water canner is sufficient. But if you want to expand your options beyond these items, a pressure canner is probably worth the investment.