You have a classic round face if the width of your face at your cheekbones is roughly the same distance as the length of your face. You likely have soft, non-angular features and "chubby" cheeks. You may find you have more of a round face when you carry extra weight, but keep in mind many classic round-faced women are super-skinny and still have non-angular features, like Cameron Diaz, in the next picture.
A great option for round faces are long layers. Keep hair longer than the chin and ask for long layers cut into hair.
Gentle waves also deflect the roundness of the face, so if you have natural wave in your hair (as most women do), allow your hair to do its natural "thing." Scrunch hair as it dries so it retains some of its wave or help along the wave by wrapping hair around the barrel of a large curling iron as Kate Winslet's stylist did here