Does anyone knows what is the difference between "rockabilly" and "rock 'n' roll"?

So far NO ONE really gave any sensible answer. Everybody are writing that "rockabilly" is a mix of rhythm 'n' blues and hillbilly/country and western music which as well can be the definition for "rock 'n' roll". When someone is trying to define "rock and roll" always says/writes that it is a fusion of blues/rhythm 'n' blues with hillbilly/country/western swing type of music. Can anyone tell the difference between those two, presumably different types of music? And please, give me the definition, not examples of both genres.