To me muscle tone exercises target the slow twitch fibers -- the endurance fibers -- the ones that don't make up that much size... I kind of have a habit of using the terms interchangably... I classify exercises as mass building or toning...
I don't know if that makes sense... I sometimes talk in my own vocabulary... it is sometimes easier for clients to understand me. I mean, a lot of the women clients i train want to keep a sleeker, defined look... not "bulky" -- however, I don't want to get into the whole point that women aren't likely to just get "big" by doing heavier weights... as it isn't a convincing argument for them.
Rather, I stick with a higher rep training regimen and try to avoid exercises like deadlifts, squats (anything that could make them "wider") -- same goes with triceps on a level -- I'm not going to use board presses for a girl who is 5'10" 110lbs and just wants to add a little tone... I can get the same results by using exercises like Triceps Kickbacks...