What's the difference between Jam, Jelly, and Marmalade?

[WhyNot]

@SanVito

I don’t know why people think marmalade hasn’t got sugar in it, it has loads (at least as much, and sometimes more than, the fruit). It’s just more bitter than jam because it uses citrus fruits (not just oranges). It’s still incredibly sweet.

There is sweet marmalade, which is incredibly sweet, and bitter marmalade, which is still sweet-ish, but not incredibly so.