In the link, which the webpage content contains, <a> tag is showing the way to the resource in the web with the help of the href attribute.
URL (or uniform resource locator) defines the address in the web. A full and correctly formulated web address should look like the following:
Scheme is the type of internet service, usually it is http. Host contains information about the domain host, usually, www. The domain defined the target domain like Port defines the port number, 80 by default. The path is the folder on the server. In case the path is omitted, the document is located at the root folder. The filename is the destination document name.
<a href="news:alt.html"> is used for accessing newsgroups
<a href="ftp: is used for ftp downloading
<a href="mailto: is used for linking to a mail service
HTML head
HTML meta
HTML scripts
HTML events