How would I go about doing this?

I would like to include code from a seperate file into an html document.  Specifically, like having a variable that would count as anything written in a seperate noted document.

example:

<cover.txt>
My opinion of <title.txt> is <review.txt>.
I give it <rating.txt> and if you like it I reccomend that you view <recommendations.txt>.

Please view my other reviews
(drop down menu code)
<../../selections.txt>
(/drop down menu code)

and each of these .txt files, or whatever they are, would contain the html which it would include in the pages.

The goal of this being to update multiple pages because they all depend on one file.

So please, how do you do this?  I hunger for knowledge, and don't know how to word my google searches.

OP, meet Server Side Includes:

Server Side Includes (SSI) is a simple interpreted server-side scripting language used almost exclusively for the web.
The most frequent use of SSI is to include the contents of one or more files into a web page on a web server.

<!--#include virtual="../quote.txt" -->

Apache, lighttpd and IIS are the three major web servers that support this language.