Being from the UK, I tend to call fruit preserves “jam”. Being in the US, most folks call them “jelly”. Both useages are, in fact, wrong, and here I explain why. The approximate definitions, in ascending order of fruitiness, are roughly as follows.
Jelly is the thickened (with pectin), sweetened juice of fruits or vegetables. Ideally it should be crystal clear, no pulp or seeds. If the fruit juice is thickened with gelatin instead of pectin it forms a dessert called “jelly” in British English and “jell-o” in US English.
Jam is made with the pulp and juice of one fruit or vegetable, sweetened and thickened either with its own pectin or with added pectin. There should be no lumpy bits, the fruit/vegetable pulp should be evenly distributed through the jam. A fruit spread is a jam made without added sugar.
A conserve is a jam made with fruit stewed in sugar. There should be recognisable pieces of fruit in the conserve, or whole fruits if they are small like berries. This style is also sometimes called “whole fruit jam”.
Fruit butters are the whole fruit, gently cooked, then aggressively cooked down until it has a texture similar to dairy butter. Most recipes talk about cooking rapidly on a stove top in a non-reactive pan – I use a crock-pot (slow cooker) with the lid off instead, as this allows the fruit to cook down and thicken without a risk of burning.
Marmalade is made with the whole citrus fruit, chopped or sliced peel, and sugar. Citrus fruits are usually rich in pectin. A lot of commercial pectin sold for home preserve making is derived from the white pith that lies under the skin of the fruit. Most marmalades sold in stores are orange, but any citrus fruit can be used to make a marmalade, such as lemon, lime, or grapefruit.
Curds are off in their own category. They are a dessert topping made with egg yolks, sugar, fruit juice, and zest, gently cooked until it becomes thick and and spreadable. They are also delicious and well worth making!