Besides being a playful abbreviation of the word "jealous" (just me?), jelly is not the same as jam, which is not the same as preserves, though they’re all in the same family. Marmalade is also in that family, and so is compote, but in a cousin-twice-removed kind of way. We’ve broken it all down for you below, the way fruit breaks down to make delicious, delicious jam.
First things first: All of the above are forms of fruit cooked with sugar, an acid like lemon juice or vinegar, and occasionally pectin. Pectin occurs naturally in fruits, but low-pectin fruits like strawberries and apricots often need a bit of a boost in order to set into the jam-like texture we all love. That’s when powdered stuff comes in handy. Once cooked this way and safely canned, preserves can live on shelves for months, waiting to add the taste of a July plum to a January piece of toast.