html5 elements confusion

I'm slowly making the transition from XHTML1.1 to HTML5 but I'm having trouble getting my head around the new elements and when they should be used. Obviously <header>, <nav>, <footer> & <article> are self explanatory but <section>, <aside> are slightly confusing.

a document might look like this:

<div id="page-wrap">
  <header>...</header>
  <nav>...</nav>
  <!-- What about the content? -->
</div>

So below the <nav> you would have the content section, would this be开发者_开发知识库 <div> or <section> and why isn't there a <content> tag?

My next issue is with a sidebar, after research I found that <aside> shouldn't be used for a sidebar, but I also read that recently the specification has been updated to include <aside> for sidebars if outside of the <article> tag. So again why isn't there a sidebar element.

My confusion spans from the presence of <header>, <nav>, <footer>. Almost every website I have ever made contains 5 sections; Header, Nav, Content, Sidebar & Footer so why is there only specific elements for 3 of these?

Hope you can clear things up for me.

In my opinion a sidebar is in the realm of presentation(css); so it was not included in the html5 specification.