A long black was supposedly created in Italy to please American tourists who would ask Italian baristas for a large cup of black coffee, similar to filter coffee that is more typically drunk in America.
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.
A long black has more crema than an Americano. This is due to the hot water being added after the espresso, as opposed to an Americano, which is the other way around.
The fact that a long black is a stronger coffee with more crema makes it a desirable drink for coffee lovers. Due to this, the long black is growing in popularity all the time, especially in the third wave and speciality coffee circle.
A short black, on the other hand, is another name for espresso.