Pectin is the jelling agent that many fruits have and that makes them good candidates for jelly and jam. Apples and quince are high in pectin, while others, like pears and some berries, have very little. So when you want to make a firm jam you either have to add commercial pectin or pectin jelly from another fruit.
Some artisan jam makers (notably June Taylor) make their own pectin. At Lindy's Toast, Lindy shows how easy it is to make apple pectin jelly - it just takes a few unripe apples, some sugar and a lemon.