Jam, Jelly, Marmalade, Preserves
Jam
Properly, the term jam refers to a product made with whole fruit, cut into pieces or crushed. The fruit is heated with water and sugar to activate the pectin in the fruit.
Jelly
In the U.S. and Canada, the term jelly refers to a type of clear fruit spread consisting of firmed fruit (or vegetable) juice made with pectin.
Marmalade
In English-speaking usage “marmalade” almost always refers to a preserve derived from a citrus fruit
Preserves
The term Preserves is usually interchangeable with Jam. Some cookbooks define Preserves as cooked and gelled whole fruit (or vegetable), which includes a significant portion of the fruit.
I always get them confused.
(via Fruit preserves)