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 an Americano.
Short black is an Australasian synonym for the basic espresso.
See also 
- Americano (coffee)
- Lungo - espresso made by allowing more water than usual to pass through the grounds
- Flat white