_root v.s. _global

Can someone explain to me the difference between _root and _global when dealing with variables or point me in the direction of a good explaniation on the web? I feel like there is something basic I am missing. For example the other day I was working on making some scenarios. In a nut shell I was setting up dynamic text boxes where the text would changed based on what the student selected (via buttons).

When the varible was changed (by clicking the button) the text wouldn’t refresh when it was a _global, but would refresh when it was _root. Everything is working fine but I’ve been trying to understand the “why” it works this way. I’ve looked around on line but can’t figure it out.