One of the main reasons to learn about Mosaic and WWW is that it subsumes other Internet tools. In another words, you can use Mosaic to search Gopher databases and WAIS indexes, retrieve files with FTP, or log in to remote systems via Telnet. This flexibility to use a wide variety of information formats relies on the notion of a uniform resource locator, or URL.
You can think of a URL as a generalization of the concept of a file name. Within a given computer system you access information by its file name. To open a file for editing you tell the editor the name of the file you want to open, or to save data from a program you tell the system the name of a file in which to store the data. A simple file name will not suffice when specifying the location of information on the Internet because there are different kinds of computers, each with their own file name conventions, and there is a wide variety of types of data and methods for accessing it. For example, a WAIS server uses an index of files, sort of the electronic equivalent of a library's card catalog, but Gopher and FTP access files directly by name.