Http://www.classzone.com is an example of what?

The string beginning with http and naming a web location is a Uniform Resource Locator that points to a site.

An access code is a password or key for entry, not a web address.

A directory organises files or lists sites by category and is not this address itself.

A server is a machine that hosts and delivers pages, distinct from the address used to reach it.

Here http names the protocol and www.classzone.com is the domain being requested.

The http prefix tells the browser to use Hypertext Transfer Protocol to fetch the page.