what is use of header tag in html 5?

Answer 1

The opposite of <footer>, search google /: …

Answer 2

it is used define headlines or branding lines

Answer 3

You use header when:
You have an article or whatever and that article contains a title, an brief comment (that's a header)

<header>
<h1>How superman is saving your ass</ h1>
<p>Superman is everywhere, you can find him...</p>
<img src ="www.supermannaked.com/image.jpg" />
</header>

You can have more than one <header> tag