Ajax content?

Hello, I wonder how simple or complicated it would be to make the content load with ajax.

If I’m not missing any, I think the possible standar pages are:
– index.php
– search.php (i usually copy here same index.php code)
– page.php
– 404.php
– image.php (i not use)

my intention is to avoid reloading header and footer, although in my particular case, all pages are the same until reaching #content div.

I know some about JS and I have seen ajax documentation, but I don’t know how to pass the query parameter (categories, tags, or content choosen to show).

Any idea is welcome.
Thank you!