The address of a webpage, like "http://www.unitarian.org/pstar/staff.html", is called a URL (Uniform Resource Locator). It specifies:
When you type a URL into the address window in your web browser program (like Netscape or Internet Explorer, the two most common browsers), that program (running on your computer) assembles a request using commands in the http protocol and sends it to the Internet, over whatever communication connection you have set up: a phone line over a modem, your company's own network, a T3 line, etc. Once the request reaches the network of interconnected computers that we call the Web, it will be routed first to a "Domain Name Server", one of zillions of computers that keep track of domain names and can translate "unitarian.org" into the computer address of the computer hosting "unitarian.org". Then the request will be routed to that machine (owned by MTN in our case). The MTN computer will find the home directory for "www.unitarian.org" and the "pstar" subdirectory and the "staff.html" file and transmit the file's contents back across the communication channel to your computer, where your browser program will display it for you by decoding the HTML mark-up commands in the text.
The HTML commands may call for pictures to be displayed, by specifying the URLs of the graphics files (usually GIF or JPEG files). In this case the browser will automatically make additional requests to the Web for those files as well. (Graphics files are typically bigger than text files, so take longer to send back to you on the net, whence the usual pause for images.) The HTML commands can also turn some of the text into "links" by associating with the link's text a URL for some other file; when you click on the text, your browser will request the associated page from the Web just as if you had typed in that URL directly. Many browsers will show you the URL associated with a link as you pass the mouse cursor over the link.