What is a URL:
A URL (Universal Resource Locator) is the most basic information about where a web page is located on the web. It includes information about which web server the page is stored on, in which directory it is located in, its name and the protocol we will use to fetch it.
Absolute links:
typing the whole address out, such as:
http://cas.la.psu.edu/cas483/index.htm
Email link:
<a href= "mailto:ellen@psu.edu">mail me</a>