Question about weight lifting? What am I exercising by doing this routine?

A muscle is called a "quad-cept" or "bi-cept" because it has bi=2, quad=4; heads=cept.

Shoulders have 3 heads, front, back, and middle. With good form you are working all three heads of the shoulder with a focus on the middle one. But not to be a ****, however, you're form sucks. I imagine you feel it more in your side-abs more than shoulders.

What I would do is sit on the bench and balance it out so that you don't sway like that when you lift. You are aiming for the vicinity of what's called a isolateral military press. Another way to do it is with the full bar and use both arms at once. That will help you balance and isolate the shoulders better.

As a rule of thumb you want to isolate muscles. if you are working biceps, the only thing moving should be your bicep. And you'll know what you are working when you feel it in that muscle. The trick, as per Victor Costa (see link) is to think about using the muscle vs moving the weight.

https://www.youtube.com/user/vicsnatural/videos