A client program for the World Wide Web, which can be defined as all URL-addressable content. The most important quality of a web browser is HTTP support, HTTP being the protocol most web content is served on; but most WWW browsers also support FTP, NNTP, sometimes Gopher or WAIS, or SMTP (for mailto: URLs), and possibly other protocols.

The first operational WWW browser was written on the NeXT platform in its native programming language, Objective C. The functional code base, libwww was ported to C and used as the basis for a simple HTTP server (CERN httpd), and the CERN line-mode WWW browser for the Unix platform.

It was as a telnet interface to this line mode browser that the WWW first met the world-wide public eye.

Soon afterwards, some people at NCSA realised its potential, and they responded by creating a Motif based graphical browser: Mosaic. This caused the Web to catch on in a big way, prompting the development of other browsers such as Cello (for the MS Windows platform), lynx (which pre-existed, using its own hypertext format, but was adapted to support HTML and HTTP), and the W3 browser for Emacs. The Windows port of Mosaic was probably the last push to make Internet support indispensable on the average PC.