The word "lungo" literally means "long", and the "lungo" gets its name from the way it's made. It is made with a so-called "long pull" (slower and more voluminous extraction) of espresso. It's prepared with the same amount of finely ground coffee and twice the water of a normal espresso shot.
A single serving is about 2 ounces, which is roughly the same as a typical doppio (a double-shot of espresso). To learn about other similar drinks and to know more about how espressos are made, check out these espresso terms.
Since it's a bit like a watery espresso in terms of its ingredients, a lungo may sound similar to a Caffé Americano. However, its unique processing results in very a different flavor. Compared to an espresso has a less strong taste (because it is made with more water), but it also has more bitterness (because the extraction process takes longer and pulls more bitterness out of the grounds). Compared to a Caffe Americano, it's more compact and (usually) stronger and more bitter.
Many people who prefer more bitter coffee love lungos, because lungos are bitterer and bigger than most espressos. Try one at your local coffeehouse to understand how extraction impacts flavor, or for a pungent, bitter and slightly-larger-than-normal shot.
Other Words for Lungo: In France, a lungo is known as a Cafe Alonge.