Orange marmalade is a cooked down mixture of bitter orange (also known as Seville orange - citrus aurantium var. aurantium) peel, often suspended in a clear jelly made from the juice of the bitter orange and thickened with pectin cooked out of orange seeds. Sweet oranges have too much juice and not enough pectin to properly set a marmalade made from them.
Orange marmalade has an intense sweet-tart flavor with a perfumey-woody orange note unique to bitter orange.
Bitter oranges have ephedra-like compounds in them, and may also interfere with statin drugs in the way that grapefruit juice does.