It used to be that the Tuber melanosporum, a prized - and expensive - treat, was only available around this time of year. Harvested during the cold months in Europe, primarily in France (the ones from Perigord are particularly coveted), the gnarly, fragrant tuber is usually referred to as the black winter truffle, to differentiate it from the black summer truffle, which is far less flavourful.
The European truffle is still prized and expensive, but because the tuber is now being cultivated in Australia - which has a growing season that's the mirror opposite of Europe's - fresh black winter truffles are now available for about six months of the year, rather than for three.
Tuber melanosporum are often called the "black diamonds" of the kitchen. Like the even rarer and more expensive white truffle, the tubers are sniffed out by trained dogs. Truffles grow underground, near the roots of certain types of tree, including hazelnut and oak. In Australia, it took considerable trial and error before growers figured out how to "plant" the tuber spores. Truffles Down Under have predators that their French counterparts don't need to contend with - kangaroos and wombats like to eat them, and Antipodean growers put fences around their trees so the 'roos can't get at them.
There are actually many varieties of black truffle - in addition to the summer type, there are also Chinese black truffles, which have some of the fragrance but very little of the flavour of Tuber melanosporum.
Unlike the white truffle, which is very delicate and needs to be shaved over a dish just before being served, black truffle is cooked with the other ingredients. As with many things, fresh is best, although they can be frozen (which keeps the flavour) or canned (although these are not nearly as good). Many types of truffle oil are truffle in name only - they're made of chemical compounds that give a vague taste of the real thing. If you're going to use it, add truffle oil in extremely small quantities.
While the distinctive, strong taste of black truffle goes very well with meats, as you'd expect, it's also surprisingly delicious with many types of seafood, especially shellfish. Black truffle and foie gras is a classic combination, and the tuber is often added to foie gras terrines and patés.