What is the difference between jam, jelly, preserves, and marmalade?

These are regulated categories in the United States rather than loose descriptions, with the FDA defining standards of identity for fruit spreads including minimum fruit content. Jelly is made from fruit juice only, strained to remove solids, which is why it is clear and holds a firm, sliceable set. Jam uses crushed or chopped fruit, so it contains pulp, spreads more softly, and is opaque. Preserves contain whole fruit or large pieces suspended in syrup or jam. Marmalade is made from citrus and characteristically includes the peel, which supplies both the bitterness and much of the pectin.

The setting mechanism is the same across all of them and explains why recipes are fussy. Pectin, a carbohydrate in fruit cell walls, forms a gel network when combined with sufficient sugar and acid at the right concentration. Fruits high in natural pectin, including apples, citrus peel, and quince, set readily. Low-pectin fruits such as strawberries and peaches need added pectin or a high-pectin fruit blended in, which is why so many strawberry jam recipes include lemon juice. Sugar is not only sweetener here but a structural ingredient that binds water and permits the gel to form, which is why reduced-sugar versions require specially modified pectin.

Adjacent terms worth distinguishing. Fruit butter is cooked down slowly without added pectin until thick and smooth, as in apple butter, and contains no dairy. Conserves typically include multiple fruits plus nuts or dried fruit. Compote is fresh or dried fruit cooked in syrup and served as a loose sauce rather than preserved. Chutney adds vinegar and spices and moves the whole thing savory. Products labeled fruit spread rather than jam or jelly are often sweetened with juice concentrate instead of sugar and fall outside the standard definitions, which is why the label wording is a reliable clue to what is inside the jar.