There are different ways to enjoy food and keep them, making jam is one of them. Read this article that discusses types of jam and their differences.
Thinking about “types of jam” is something of a knotty problem,” and when asked, “Why is ‘thinking about types of jam’ a knotty problem?” the answer doesn’t help either: “That too is a knotty problem.” As far as unsatisfactory answers go, it’s a humdinger, I accept, but then “types of jam” really does throw up some of the complexities of the language.