There's beeen links in the thread to a Marmite FAQ site, and also to a song I wrote about it. But there they are again to save time:
Incidentally, Marmite's the brand name, but most supermarket chains in England make their own variety of "yeast extract" which tastes the same. The Australians have an equivalent called Vegemite which is similar, but with a slightly different flavour, and they sell that in Tesco's as well, (also placing it with the jam, since you spread it on bread and it doesn't have any meat in it).
They key thing to appreciate if you ever try Marmite (etc) for the first time is that it might look a bit like chocolate, but it doesn't taste anything like it like it, and a little of it goes a long way. That may be where a lot of the confusion comes. Think peanut butter distilled to the strengtgh of chilis and you might approach the taste with the right mindset to enjoy it...(Not that it tastes like that either.)