I won't go into the dynamics of the header function programming, but here is what I use mine for. I have ads and highly dynamic content on my website and I do not want this information cached for two reasons. A) if its cached, then I don't get credit for displaying a banner and b) sometimes people will miss out on up to date information because old information is cached on their system. they would manually have to hit refresh every page if i didn't have "N0-cache" specified in the header.
also, the header is very useful for redirecting people to another website or page within your website. this becomes very useful in more advanced programming when you want to track what users click on what links in your website. Header is very important and very useful. You should read about it using the link provided by 'PAUL' AND you should visit wc3.org to learn how header is used in general in HTML.
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