Upfront Editor versus Upfront Builder

Hi-

I am working to create a new theme for my client. I think what I have discovered is that I am working in Editor versus Builder. Please clarify for me what the difference is and how and why each is accessed. For example, if I use the top menu bar Upfront, I have learned this is the Editor. Whereas, if I use the Dashboard option Upfront, and then select Builder, I have learned this is the Builder.

I believe that when I thought I was saving changes to my theme by using the Editor choice, I was merely making modifications to that theme and not updating it. Is this true?

Best Regards-

Becky