Ron,
!--#include is actually part of two different things.
1. <!-- Text between these "arrows" is a comment and will be ignored by
browsers. -->
HTML comments are useful when you want to annotate your code, but don't want
those messages displaying in the browser window. They're still visible in
the source, so don't include anything top secret.
2. #include virtual="/path/to/filename.htm"
This is Active Server Pages (ASP) code to include the content of one page
into another. The correct syntax for this declaration requires it to be
placed within an HTML comment as shown in your original post:
<!--#include virtual="/left_navigation.htm"-->
Here's an intro to ASP includes that you may find helpful:
http://www.w3schools.com/asp/asp_incfiles.asp