Americano and Long Black are variations of the same idea. You'll sometimes see them on coffee menus, but you'll also hear them referred to by other names like "coffee crema," "allongé," or simply as "regular coffee" in various languages. Basically, it is diluted espresso.
Americano comes from the Italian for "American" or "American coffee" and comes from the fact that people want something like filter coffee in cafes that only have espresso machines. Think about the coffee you had the last time you were in an American diner: a rather weak-tasting filter coffee that you could sip on all day. Americanos are made by diluting a single or double espresso with boiling water from a machine.
In Australia and New Zealand, the Americano is called a "Long Black" - it even follows the same naming scheme as the Flat White! It contrasts with a "Short Black," which is a simple espresso. The special thing about a Long Black is that baristas first pour the hot water into the cup before pulling the double espresso over it. This preserves the crema and makes the coffee look a bit more beautiful.
