It is a concentrated yeast extract that is a by-product of making beer. It is very high in B-group vitamins and salt. We use it primarily as a spread on toast or sandwiches. The most common thing to couple it with (other than butter or margarine) is cheese. Most of us here in Australia started eating it as babies. Some parents even put some on their baby's dummy to keep them happy.
You can buy Marmite and Vegemite around here, but nobody does.
I said "Do you speak-a my language?"
He just smiled and gave me a Vegemite sandwich.
My mom tried feeding me the stuff when I was young, didn't go over too well.