How to get a pop-up box (not a new window) appears on mouse click?

I want to do something, but i dont know how clear i'm going to explain
it. Please help me with this matter. Following is what i want to b
done.

i'm doing a web site which has several items (which suppos to b links)
listed one after one.

When u click on each item, there is a description to b appeared over
the page. What i want is, this description page (or rather a box)
should appear over the excisting details of the page, and also i want a
close button (something like this to click >>> CLOSE [X] ) on it too.

This shouldn't b another (pop-up) window, but some kind of a box which
come on top of the page, and on a possition a give.

i would b appreciate If anybody can help me with this issue.

Thanks a lot...!!! :)

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