Explain the divs – header, masthead, branding… twenty ten

I’ve been dorking around with a header image that has a transparent top and have been fighting with how best to work that in, along with centering the access div… and am just now feeling defeated . 😉

So I have a general (probably dumb) request… can someone explain to me what the header, masthead and branding divs are for?

I can see the ‘branding’ div pulls in the changeable header image, the site title and site description.

What is the masthead and header div for?

And why is centering the access div or slipping in another div below it so difficult? (rhetorical question, it just felt better to type it ‘out loud’)
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