looking for examples -- link to insert the "content" into a given area

Hi.

Combination php/js issue.

Trying to find example/explanation on how to insert content froma link into
a given area.  Just as a test, did a test webapp, and tested
  http://1.2.3.4:/aa.html and the entire page was displayed from exa,img
the page source. . So, I was curious as yo the process if I had a menu
item, that returned content, if I wanted to insert that content into an
area on a page. How to accomplish this.

Searching, haven't found an explanation yet.

thanks