TeX's model of the world is (broadly speaking) that the author writes text, and TeX and its macros decide how it all fits on the page. This is not good news for the author who has, from whatever source, a requirement that certain things go in exactly the right place on the page.
There are places on the page, from which things may be hung, and two LaTeX packages allow you position things relative to such points, thus providing a means of absolute positioning.
The textpos package aids the construction of pages from "blobs", dotted around over the page (as in a poster); you give it the location, and it places your typeset box accordingly.
The eso-pic defines a "shipout picture" that covers the
page. The user may add picture-mode commands to this
picture, which of course can include box placements as well as the
other rather stilted commands of picture-mode.
(Eso-pic requires the services of everyshi, which
must therefore also be available.)
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=abspos