Sometimes, I wonder why anyone would buy those bottled jams in the supermarket during the summer. We have an abundance of cheap fruit, and it's so easy to make at home - they taste superior as well. Problem is, preservation.
Well, preservation here comes in two forms - the sugar and the bottle. Jams and preserves are so sweet because of the amount of sugar used, and that's also because the sugar prevents bacteria growth. Like us, bacterial cells need water to survive, and if the concentration of sugar is high enough, it sucks the water right out of them (look up "osmosis"). Bottling them also serves to heat kill many of the bugs before they even get a foothold.
But really, what does it take to make a jam? Just mix fruit with some sugar, and heat it for a while. If you barely cook the fruit, it's a compote. Cook it until it breaks down, it's a jam. Or a preserve if there's enough pectin in the system. And then, you can keep them around in the freezer. In this way, one can make jams that aren't quite so sweet since you don't need the preservative effects of sugar.
It's so easy, you can probably whip some up for breakfast to go on the toaster waffles.