A fool is made by stewing or mashing fruit into a sweet purée, then folding the fruit into either double cream or custard. It is served cold.
In North America, where the cows seem genetically unable to produce double-cream, whipped cream is used as a substitute.
In the UK, the fruit used is often gooseberries; in America, the fruit is more likely to be blackberries or blueberries.
Modern-day versions can use a thick yoghurt such as Greek yoghurt, or a fresh cheese such as quark, in place of the double cream or custard. The fruit can be anything now, even cranberries.
Fools appear to date back to the 1500s. Fools also became popular in tropical British colonies, where the custard could be made from coconut milk, and the fruit might be mango.
The most plausible origin of the word “fool” in this context is that it comes from the French word “fouler”, meaning to mash. Alan Davidson (The Penguin Companion to Food. London: The Penguin Group, 2002. Page 374) notes that while it is “thought” that “fouler” could be the origin, dishes of mashed fruit with cream in some form have been around aeons before the term “fool” would have been coined.