When reaching for a jar of old fruit and sugar, you have a few options. You have jelly, which is great for sandwiches, jam for muffins, and preserves for eating with a spoon when no one is looking.
Anyone with eyes bigger than their stomach at a farmer's market can tell you that fruit doesn't last very long. To keep the sweet stuff around longer, many people in the 19th century resorted to jarring their fruit with sugar to ensure fruity goodness even in the colder months. Today, these jarred fruits are enjoyed alongside fresh fruit that's available all year round. All of these concoctions are similar but have important distinctions. What it really comes down to is cooking method and consistency.
Preserves is a term often used to describe all fruit spreads, but can also refer to a specific type. It's made up of large chunks or whole fruits that are cooked in sugar until they are soft and then put into a firm jelly or less-gelled syrupy base.
Jam is made out of mashed fruits, sugar, and a little bit of pectin, a substance that makes everything gel and stick together. Jam also contains fruit juice, giving the sweet goo an almost opaque appearance. While not chunky like preserves, jam is typically more solid and substantial than jelly. According to the FDA, for a jar to be labeled as jam, it needs to have a "soluble-solids content" of 65 percent or greater.
Jelly is the most translucent of the bunch and more likely to be found in a child's lunchbox. It uses more pectin than jam in order to obtain its gel-like quality. Generally, the fruits are cooked much longer when making jelly. The essence of the flavor is captured as a result of longer cooking times. There are no bits of fruit in jelly—the cooked fruit juice is filtered through a muslin stockinette or jelly bag to give it that see-through appearance.