The answer will depend on who you ask but if you ask me, and customers frequently do, a latte, flat white, café au lait, café con leche, etc etc are essentially the same drink in different languages and on different continents: espresso coffee with hot milk.
The difference really comes down to what they are served in and the person making the coffee. At Lantana, flat whites are served in ceramic cups and lattes in tumbler glasses. Both have two shots of coffee but we make the latte on a shorter double shot (a ristretto) which means flat whites are stronger, and, as the name suggests, are made with less foam than a latte. If you want to get a more technical answer from people who know what they are talking about look at toomuchcoffee.
Tim Dunn (ex proprietor of Pushka in Melbourne and now barista in residence at Lantana) shows the difference between Lantana's lattes
and flat whites