Vegemite is made from leftover brewers' yeast extract, a by-product of beer manufacture, and various vegetable and spice additives. The taste is difficult to describe but is extremely salty and slightly bitter, and as might be expected, malty. While highly popular in Australia and New Zealand, it has never been successfully marketed elsewhere. It is notorious for the dislike it generates amongst some foreigners. Vegemite is not liked by all Australians - some find it far too salty to be palatable - but it remains an iconic symbol of Australia. It is available at some supermarkets and specialty food shops in a number of other countries.
I think the stuff tastes like crap dipped in salt. But some like it.