Both are espresso and hot water; the difference is the order. An americano is the shot topped up with water. A long black is the shot poured on top of the water, which keeps the crema and makes it a little shorter and stronger.
The differences
Order: water on espresso (americano) against espresso on water (long black).
Crema: a long black keeps its layer of crema; an americano breaks it up.
Strength: a long black usually uses less water, so it tastes bolder.
Where: "long black" is the name in Australia and New Zealand; "americano" almost everywhere else.
