Americanos and Long Blacks are variations of the same thing, and you’ll sometimes see any number of them on coffee menus, as well as several other names in different languages like "Kaffee Crema", "Allongé", or just "normal coffee". At their core, they are diluted espresso.
Americano refers to “American coffee”, and stems from people wanting something like filter coffee in cafés that only have espresso machines. Think of the coffee you had last time you were in an American diner: low-strength, bottomless filter coffee that you can just sip away at all day long. Americanos are made by watering down a single or double espresso with boiling water from the machine.
A long black is just what people call americanos in Australia and New Zealand — it even follows the same naming scheme as the flat white! It sits in opposition to a short black, which is a single espresso. The one thing unique to a long black is that baristas might fill the hot water into the cup first, before pulling the double espresso on top. This helps to preserve the crema, making the coffee look just that little bit prettier.