Server Side Includes
(SSI) are codes you can add to your HTML document that tell the web
server to include other content with the document being served. For
example, you can use an #include statement to display the same header
and footer on your pages automatically; if the information changes, you
can save time by editing a single header.html or footer.html file. You can also use SSI for graphics such as logos or image maps that should appear on multiple pages. Pages that use SSI (#include statements) need the .shtml extension. Included files can have other extensions (e.g., .html or .php).