Under U.S. federal food rules there is no legal difference: the FDA defines fruit “preserves” and “jams” in one shared standard of identity, so the same product may be labeled either way.
That standard, 21 CFR 150.160, covers viscous or semi-solid foods made from fruit and a nutritive sweetener such as sugar, concentrated with or without heat. For Group I fruits such as strawberry, raspberry, blueberry, and grape, the mixture must contain at least 47 parts fruit for every 55 parts sweetener by weight; for Group II fruits such as peach, apricot, pear, and plum, the minimum is 45 parts fruit to 55 parts sweetener. The finished jam or preserve must reach at least 65 percent soluble solids. Pectin may be added only to make up for fruit that lacks enough natural pectin, and spices, acidifying agents, and preservatives are also permitted. The label must say “Preserve” or “Jam” together with the fruit’s name, or “Mixed fruit” or the fruit names for blends of two to five fruits.
Source: 21 CFR § 150.160, Fruit preserves and jams (Cornell Legal Information Institute)
Image: Placing lids on strawberry jam by Nutrition, Food Safety & Health, CC0, via Wikimedia Commons.