Many recipes for preserves, including jams and most especially jellies, call for the addition of pectin. So what is pectin, anyway, and why is it an important part of preserving?
Pectin is a starch (a heteropolysaccharide, if you must know) that occurs naturally in the cell walls of fruits and vegetables. It is, in fact, the very thing that gives them structure. When cooked to a high temperature (220ºF) in combination with acid and sugar, it forms a gel.
This is what gives jams and jellies their set when they cool.
Some fruits, like apples and quince, are naturally very high in pectin; this is why they are very firm. The rinds, seeds and membranes of citrus fruit are also very high in pectin -- up to 30% by weight. This is why marmalades are made from citrus. (Fun fact: The word "marmalade" comes from the Portuguese marmelada, for quince paste, derived from marmelo, for quince. It wasn't until the 17th century in England that citrus became available enough to take over the meaning of the word.) Commercial pectins are usually made from citrus rinds.
Other fruits, especially very ripe ones, are less endowed with pectin. Think strawberries and raspberries, which squish easily. For these fruits, without added pectin, getting a set may require adding lots of sugar, cooking for excessively long times, or both.
To find out how much pectin is in your fruit, try this test.
Combine one tablespoon of grain alcohol and one teaspoon of your fruit juice. If it sets up firm, it's high in pectin. If it becomes a loose, gelatinous mass, it's medium on the pectin scale. If it doesn't set at all, or forms slivers of gel, it's low in pectin.
What kind of pectin you use matters. Dry pectin comes in multiple forms, tailored to the amount of sugar in a recipe.
Liquid pectin is similar to the regular dry pectin, but pre-dissolved to avoid clumping. Pomona's Pectin is a popular brand of a type known as low methoxyl pectin, which combines with calcium instead of sugar to create a set, and so is good for low- or no-sugar preserves. And you can even make your own pectin, using citrus or apples.
Each type of pectin behaves differently, so it's best to follow the recipe you are using. If you find the set too hard or too soft, you can always adjust the amounts accordingly. In some cases, different kinds of pectins can be substituted, but it's important to know which and how.