Example:
One cool page to visit is the <A HREF="http://www.whitehouse.gov">White House</A>.This is shown in the browser as:
One cool page to visit is the White House.
Notice the words White House are blue and underlined. This means that they are active. In other words, some action is associated with White House. That action is invoked when the user clicks the mouse on White House.
The action is defined by the URL that is assigned to the HREF attribute. Any valid URL is allowed as a value for the HREF attribute (mailto, ftp, http, etc.).