It is common understanding that a round peg cannot be fitted in a square hole. In just the same fashion, there is no common hair style for all face and structures. Each person depending on the structure of his or her face and frame would need to address their hair in a different way.
A round face is typically one that has no real axis to work around. The best way to deal with this face type is to try and form some structure using the hair style that will then help accentuate the features of the face.
Try using styles that will help make the face look slightly longer. Short hair suits this face type very well. Try using styles that stay very close to the face. This helps give the face a perception of greater length. If you have a slightly longer length, you could opt to layer the top starting with very short layers that fall over the face, making the crown look much longer and adding height to the face. Avoid using hair that falls over the face on the top it will make the face look smaller. Should you want hair that styles at the very top, try using a short fringe to good effect.
A complete no-no is curly hair. The curls highlight roundness and make the shape of the face stand out even more. Stay away from pulling the hair back completely as well.
Colouring the hair is also a viable option. Colour just the ends of the hair when it is layered to give it a more tapering look. This will also add to the perception of length.
The idea is not to get weighed down by limitations you might feel you have. Work with and around them and you will see how they contribute to giving you a look you could never imagine you had.