These are simply text files containing HTML tags. Note that the web-server has done nothing more than send the file to the browser. It has not read any of the file's content. As such, this is the quickest way of serving a page.
S stands for server-parsed.
These look like HTML comments, but have special significance. If the web-server finds an SSI, it converts the instruction into its output. So if the instruction is to print the current date, the webserver will datestamp the page at that point. Various instructions like this are available. ASP is a Microsoft technology called Active Server Pages.