As the term has come to be used in the United States since approximately 1985, a latte consists of one or two shots of espresso and about three times as much hot milk, topped with a small amount of milk froth. A latte has more milk than a cappuccino, and has a weaker, milkier taste. Lattes should be prepared by pouring milk and coffee simultaneously, from either side of the drinking vessel, which ideally should be a tall, ceramic mug.
The evolution of this term (and this particular form of the beverage) is relatively recent and probably dates from the spread of the 1980s Seattle coffee craze to the rest of the United States (and beyond) via the growth of Seattle-based Starbucks. As recently as 1980, if you ordered a "latte" in an Italian coffeehouse in San Francisco, you would expect to get a glass of milk. If you ordered a "caffè latte," you would get a beverage with far less milk in it than what has now come to be called a "latte."