A teapot and a kettle are both kitchen tools used for brewing tea, but they function differently and have different aesthetic values. A kettle is a handled pot with a spout that is used for boiling water on the stovetop (or it can be electric). Kettles can be made from a wide array of materials such as stainless steel, copper, or aluminum. Kettles do not steep tea, but rather just heat the water required to steep tea in either a cup or teapot.
Unlike a kettle, a teapot is not meant to be heated by a direct heat source. Rather, it is meant for holding hot water and steeping tea leaves. Like a kettle, it has a handle and a spout, but it also has a built-in or removable strainer to hold tea leaves. Since tea can be served directly from a teapot, these pieces are considered to be serveware pieces. In comparison, kettles are generally considered to be cookware.