The round (often red when turned on) plates on top of your stove where you put your pots and pans to boil/roast stuff. They can also be separate from a stove.
Does "hob" refer to the entire stovetop or to a single one of the (often four) rings?
How would you fill the gap in this sentence? "On the stovetop, you can cook food on four ____ at once."
That would be "kogeplader" in Danish: "På komfurpladen kan man lave mad på fire kogeplader på én gang."
As xneb says, the (four) rings are called hobs.
But I think I would also call the whole top of the oven a hob. If I was looking to move into a new house I might ask if it had a gas or electric hob, meaning the whole thing rather than one ring. I don't know if it's strictly speaking correct to do so, but I can't think of anything else to call it. I wouldn't call it a stovetop, which sounds American. Anyway to me a stove is an old-fashioned wood-burning thing for heating a room, not a kitchen appliance.
Interestingly, when I googled hob and burner I found a well-known British shop that calls the top of the oven a hob and the ring a burner, which seems like a confused muddle of UK and US!
As an American I say I cook things on the 'stove' when I mean cooking on a burner. I only hear people use the word 'burner' when the fact that the person is cooking on the stove is already established, otherwise they just say 'stove'. So I wonder if it's also correct to use 'stove' in this situation, at least in American English.