jojasary,
You can add either a text or html file to a table, or division in your page using the<object type="text/html" data"file.html" heigh="inpixels" width="inpixels">"
heres where you put whatever code you want to show up if the file doesnt load for whatever reason.
</object>
either <object> or <iframe> would work for this though some might shun using the iframe. also, if your site runs on PHP, you could also use an include.
I would say server-side includes are a better way to pull this off (like the PHP includes maw mentioned, but no extra stuff).
Syntax is roughly
Code:
<!-- #include file="/path/to/file.html" -->
Google can clean up remaining inconsistencies.
Note that, in this case, you should not have separate <html> / <head> / <body> tags in each file. You're effectively copy/pasting content into the page, so just start with the relevant info and proceed from there. The same may be true for the <object> method; I'm not sure.