In a Nutshell
There are many classifications, but generally speaking, jelly is made from fruit juice, while jam is made from crushed fruit.
The Whole Bushel
When boiled with sugar, fruit will gel up and become sticky (fantastic for spreading on toast). Most people probably think of jam and jelly as being synonymous, but this is not so.
While the FDA has many classifications for fruit preserves, the difference between jam and jelly comes down to this: jelly contains only fruit juice. It gels up more, holds it shape better, and is much clearer and thicker. Jam is made with fruit puree and often contains chunks of actual fruit; as such, it is opaque, softer, and more spreadable.
Also: preserves are like jam, but made with whole pieces of fruit instead of puree. Marmalade is made with the peel and pulp of citrus fruit—it must be cooked for much longer because it generally contains little or no pectin, which is the chemical contained in fruit that causes the gelling effect. Pectin can also be purchased as a food additive, for jams and jellies made with fruit—mostly berries—that don’t have enough natural pectin to manage the thickening.
One other important difference? Jam is better for you. Since it’s made with actual fruit instead of only with juice, jam is much lower in sugar than jelly and has much higher overall nutritional value.