Indeed, there are two different types of home canning systems. When it comes to pressure canners vs. boiling water canners, it’s what you are planning to can that determines which canner is needed.
A Boiling water canner is basically an oversized pot, usually made of steel or enamel- or porcelain-coated aluminum, with a lid and a rack that suspends from the edge of the pot and then lowers the cans of food into the water to be submerged completely. Since the food is being heated in boiling water, it never gets hotter than the boiling point, 212˚F. The boiling-water method is used for acidic foods, including pickles, fruit, sauerkraut, and tomatoes, and the cans are usually processed 10 to 20 minutes depending on the food and the size of the jar.
A Steam Pressure Canner, meanwhile, functions much like a regular pressure cooker. It consists of a large base pot, a rack that elevates the jars from the bottom of the pot, and a lid with a vent, safety valve, pressure regulator and locking mechanism. The canned foods are heated in the pressurized steam, which reaches a far higher temperature – about 240˚F – than a boiling water canner, hot enough to destroy botulism spores and other potentially toxic bacteria. This higher-heat method is safe for low-acid foods like vegetables (such as okra, carrots, beets, asparagus, beans, peas and corn) as well as meat, poultry and seafood. Pickled vegetables, however, can likely be processed in a boiling-water canner since the vinegar solution they're in is acidic, but check your recipe to be sure.
Acidic foods can be processed in a steam pressure canner, but it is a much slower and more complicated process, since the water needs to be heated, the canner vented, then pressurized, the food processed, and then the canner must be cooled for up to 30 minutes before the jars can be removed. The entire process could take as long as 1 hour 40 minutes. With a boiling water canner, you simply bring the water to a boil, which might take about 30 minutes, and then you’re ready to process the jars of food, so it's probably easier to just process high-acid foods in your boiling water canner. Whichever canner you use, be sure that you follow the directions exactly in your recipe and adjust cooking times and temperatures according to your altitude. The National Center for Home Food Preservation is a good reference for canning instruction and information, as is the Ball website and About.com's Guide to Food Preservation. For more information on canning equipment, check out the Canning Equipment Checklist.