I am new at this malarky too and have been experimenting with server side html includes. (No need for php or asp etc.)
You simply create one file with the content to be duplicated elsewhere and call it what you like.
Then in your html doc you put the following in where you would like the included segment to be inserted:-
<!--#include file="filename.html" -->
You save this file as whatever.html or .shtml (apparently the latter is preferred by some servers, although I don't know why).
The resulting page in the browser looks like a fully formed page.
You can have many includes in one document and, if you wanted to, you could make a template containing all of the includes for future use.
I heard, somehwere along on the line, that using includes speeds up page loading, as the included data is cached.