Both are small and coffee-forward, but they are opposites in texture. A cappuccino has a tall, airy cap of foam; a flat white has almost none, just a thin, silky layer that sits flat with the rim.
The differences
Foam: 1.5–2 cm on a cappuccino against about 5 mm on a flat white.
Texture: a cappuccino feels light and frothy; a flat white feels dense and velvety.
Milk: similar amounts, but much more of a cappuccino's milk has been turned into foam.