A long black is a style of coffee, most commonly found in Australia, New Zealand and Brazil. It is now becoming available in the UK, predominantly in London.
A long black is made by pouring a double-shot of espresso or ristretto over hot water (usually the water is also heated by the espresso machine). A long black is similar to an Americano, which is made by adding hot water to the espresso shot; but a long black retains the crema and is less voluminous, therefore more strongly flavoured.
The order in which a long black is made (water first, espresso second) is important—reversing the steps will destroy the crema from the espresso shot and make a Americano.