The following short excerpt is courtesy www.boutell.com.
The easiest way, by far, is to use “server side includes” in your HTML. Server side includes are a simple way to tell your web server to insert various things at various points in your HTML page. You can use them without major changes to your existing pages. The code looks something like this:
<!–#include virtual=”insertthisfile.html” –>
However, some web servers require that you name your file .shtml rather than .html in order to enable the parsing of your file by the server in order to find server side include directives. Read more
There is also an excellent “primer” about this subject at TheSiteWizard.com
