It's a chocolate coated biscuit, with usually a cream filling - though there are different varieties (like caramel, double-choc mud cake, tiramasu, honeycomb, raspberry.... many others ). Tim Tam Wiki page
That looks pretty good. I'll have to buy some when I make my trip to australia. That probably won't be for a couple decades though, since I have to be old and rich to do it. Plus I have to hit antartica and Brazil first.