I agree with Henrik, I think that quebrar is used when you break something into pieces like a plate or a chocolate bar. Romper is used when something is not working, like a car, "mi coche está roto". Language is always so pliable though, differences are subtle, for example to say "Mary tore the paper" I would never use quebrar even though tearing a paper breaks it into pieces. I would say "Maria rompió el papel." This may be a special usage, I am not sure. In simple terms:
quebrar => break into pieces, such as plates, glasses.
romper=> to cause to stop working, something not working, like a car, a watch.
Hope it's not too confusing.