Boiling water canners are either made of aluminum or porcelain-covered steel. They typically come with perforated, removable racks and a fitted lid. The canner needs to be deep enough for at least 1 inch of boiling water over the tops of the jars.
Some boiling water canners don't have a flat bottom, but this is necessary if you are going to be cooking on an electric stove. A flat or ridged bottom will work for a gas stove. For uniform cooking on an electric stove, you should choose a canner that is no more than 4 inches wider in diameter that the heating element.
Here is a YouTube Video on how Boiling Water Canner works