Military experts, please explain to me ranks and roles.

I love the military, and regret having never joined myself.

That said, the roles and ranks tend to confuse me.

I believe I understand that the Marines are all soldiers, therefor they have no chaplains or medics so they use their brethren from the Navy to fill those roles.

In this movie, the entire defense team is Navy officers while the prosecuting team are all Marine's. So, do Marine lawyers only prosecute? Are there really Marine JAG Officers? Why wouldn't the Marine's be defended by Marine lawyers instead of Navy lawyers?

I honestly don't know how to phrase my question, but the military relationship between the Navy and the Marine's has always fascinated me as I don't clearly understand it and who decides what. They seem to almost to be one branch, but are always treated and act separately even though they function almost always together.

If some jarhead or squid could explain to me in detail how all this works, I'd be greatly appreciative.