Originally Posted by hip hop bunny hop

Plateauing can occur multiple ways; two notable ways being not increasing the difficulty of a work out and not changing up your workout. If you were doing bicep curls and continually upped the weight at regular intervals, your gains would still be retarded at some point, due to lack of variety in exercise.

And to answer your question, a simplistic equation would be.... less weight, more reps = more tone. More weight, less reps = more muscle.