The Distortion node adds harmonics to a signal by applying a sigmoid function to the signal. As the input signal becomes larger, the sigmoid behaves more like a step function.
To create a great rock distortion sound for electric guitar, run a HighPass node before the distortion, to give the guitar some tightness and a LowPass node after as a tone control. Adjust the HighPass for more of a fuzzy sound. Connect this to a good old tube amp and commence rocking.