A flat white is smaller and stronger-tasting than a latte. Both are espresso with steamed milk, but a flat white has about half the milk and only a thin, flat layer of foam, while a latte is a tall cup with a centimetre of foam on top.
The differences
Size: a flat white is usually 150–180 ml; a latte is 240–350 ml.
Milk: about 120 ml in a flat white against 200 ml or more in a latte, so the coffee comes through much more in a flat white.
Foam: a flat white has a thin layer of microfoam, about 5 mm; a latte has about 1 cm.
Shot: both usually take a double shot. Some cafés use a double ristretto in a flat white for extra sweetness.