Most stove-top ones tend to whistle when they boil. They will be made of heat-proof glass or, more usually, metal.
Electric kettles tend to boil water much faster than stove-top kettles. They may be made of metal, glass or plastic. Most have a safety now so that they shut themselves off when the water has boiled, eliminating the possibility of the kettle boiling dry and causing a fire.
Cordless electric ones aren’t truly cordless. The cord goes into the base, and the Kettle sits on the base, drawing its power from the base. But when you lift the Kettle, the Kettle itself is cordless. Cordless electric Kettles can be seen as safer, because they eliminate the possibility of a Kettle full of just-boiled, scalding water suddenly being yanked because the cord is in the wall, with hot water resultantly sloshing everywhere
Some electric Kettles have see-through sides so you can easily see how much water is in the Kettle.
When buying a Kettle, always think of trying to reach your hand in through the top of it to clean it (will a hand fit in easily?) Some Kettles will pour on the slightest slope; some require you to tip them almost 90 degrees to pour. When considering one for purchase, go through the motions of pouring with it, and look at where the spout is in relation to the handle — some short spouts are in just the right place so that when the Kettle is tipped to pour, the steam will rise vertically up over the top of the Kettle and scald the hand holding it a lobster-red.
Classically, a Kettle is a large, black vessel made of metal, usually iron. It will have a lid, and a looped handle that swings up. Some Kettles had three tripod feet; some had just a flat bottom.
These Kettles were for cooking soups, stews, sauces, etc, in large quantities.
The capacity of reproduction ones still being made can range from .7 quarts to 10 quarts. Some of the reproduction ones aren’t for cooking with, however, they’re just for “serving” in — some of them, for instance, are very small, holding just 1 cup (8 oz / 250 ml) of stuff, and essentially are novelty chip dip bowls.
Electric “cooking” Kettles are now also being made. They will come with a strainer so that you can also use them as a deep-fryer.