i really want to get great back and shoulder definition. from the back.

ya know the kind? not muscular but sexy and curvy.. what will be the best kind of thing for that
?

So you want muscles.

Read my post above about toning. There's really no such thing.

Complex, functional moves for the back/arms/shoulders will help build the muscle that you want to give that definition. Planks (which are also great for the core muscles), pushups, tricep dips, pullups, curls, skull crushers, overhead barbell press, rows ... all will help build and define the muscles.

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I love the lat pulldown machine for toning the back. You can also use resistance bands at home. Dips are fantastic and so are rows. Don't fear bulking up! It's hard enough for women to add muscle mass and on a calorie restrictive diet it's practically impossible. Photochick made a lot of other good suggestions that I also use and I've seen some significant changes.

Any type of shoulder press is great for working those muscles as well.

Yup I second Photochick... using your body 'against' itself is a great workout.

Ah the plank... it's a killer... managed 80 seconds yesterday before I hit the deck lol

Pushups are an awesome upper body exercise but I have to say the thing that has made my shoulders awesome are kettlebells.

ohh! sexy back!
here are some of my favorite back, sholder and core exercises

Back Extensions
Pendulum with Ball
Dumbbell Reverse Flys Lying on Ball
Lying Row & Rotation on Ball
upright row
lat pull down
good mornings
bicycle crunches
side plank
and yoga!