URL is a TLA for "Universal Resource Locator", which is e-speak for the kind of address you specify in order to find something on a remote machine. There are several kinds of URL's, but all have the same form:

This is an HTTP (hypertext transfer protocol) address native to the WWW. The first element after the characteristic double-slash is the Internet address of the machine where the file is to be found; the subsequent elements specify the exact filename on the machine in question; that the last element ends in *.html tells you that this is a genuine WWW resource with the text formatted in Hypertext Markup Language. Other forms of addresses begin with distinctive terms, but all can be reached from WWW-browsing software:

To avoid errors in copying these mystic runes, it is often a good idea to use the mark/copy feature of software common to all Macintoshes and to DOS machines running Windows, to copy and paste the exact series of characters from wherever you have found it to where you can store it.

Hotlists: But NOTE WELL: in Mosaic and Mosaic-like browsers, there is usually a "hotlist" feature -- if you are looking at a valuable resource on screen and want to come back, add it to your hotlist and it will be on a pulldown menu for every subsequent session on the same machine. (You may also delete obsolete items from your hotlist.)