A web browser is a computer program used for accessing the World Wide Web. A web browser retrieves and renders webpages to display information stored on a web server. The browser sends an HTTP request to the server, which responds with the requested file. Most web pages are written in HTML, a markup language which allows inclusion of other files; the browser interprets the file and makes additional requests of the server for the additional files, then interprets and displays the results for the user.
The most popular web browser today is Microsoft's Internet Explorer. Internet Explorer has come bundled with every version of Microsoft's Windows operating system since Windows 95, and is supplied free of charge. Microsoft created versions of Internet Explorer for the Macintosh, Solaris and HP-UX operating systems, but has ceased development on all three versions. The most recent official version is Internet Explorer 8, although Internet Explorer 7 and 6 are still supported.
Internet Explorer's Trident layout engine is part of Microsoft Windows and can be embedded in third-party applications. There are therefore many browser shells based upon it, such as Maxthon.
Mozilla/Gecko based browsers
The Mozilla family of browsers is based on the the open-source Mozilla Project, started by Netscape Corporation to develop a new browser to succeed its Netscape Communicator suite.
Mozilla Firefox is the second-most popular web browser today. Versions of Mozilla Firefox are available for the Windows, Macintosh, and Linux operating systems; additionally, as the browser is open-source, volunteers have ported it to Solaris and OS/2.
Camino is an alternative version of Firefox for Mac OS X, made to conform more closely to the Macintosh's interface conventions.
There are many other browsers based on Mozilla's open-source Gecko layout engine.
WebKit/KHTML based browsers
The open-source KHTML rendering engine was initially developed for KDE's Konqueror web browser for Linux.
When Apple developed it's Safari web browser, it created a fork of KHTML for Mac OS X called WebKit. WebKit is open source, and the changes are back-ported to KHTML. WebKit and Safari have now also been ported to Windows. Safari was the first browser to pass the W3C's Acid2 rendering compatibility test, is currently the third most used.
Since WebKit is part of Mac OS X and is also available for Windows, there are many browsers based upon it, including Google Chrome - which as of late 2009 had eclipsed Safari in popularity  - and the Mac-only Omniweb and Shiira.
The forth most popular browser is Opera, initially developed by Telenor of Norway. The developers formed Opera Software, and released version 2.1 publicly in 1997, at version 2.1. Until 2005, Opera was sold for approximately $25 to $35, though an ad-supported version had appeared earlier for users who did not wish to pay. Since then, it has been available for free. Opera had an early lead in compliance with the CSS standard, and claims to be more standards-compliant than its competitors. Versions of Opera for mobile devices are the default browsers on several mobile phones. Other versions have been developed for video gaming consoles such as the Nintendo Wii.
The first ever web browser, Nexus (later renamed WorldWideWeb) was developed on a NeXT computer by Tim Berners-Lee, the inventor of the web, and released in March 1991.
Tim Berners-Lee on the creation of the web
Several other browsers were developed in the subsequent two years after the invention of the internet, but the most important was NCSA Mosaic. Mosaic was developed by Marc Andreessen and Eric Bina from the NCSA, released in February 1993 on Unix, and later that year for the Macintosh (computer). The 2.0 version of Mosaic, released in January 1994, was the first browser to actually show images embedded in the text, rather than in a separate window.
Netscape Navigator was the first browser to gain widespread usage, and was developed for a variety of platforms, including Windows and the Macintosh. In response to competition from Microsoft Internet Explorer, it was ultimately distributed with a newsreader, HTML editor, and other features. However, it steadily lost market share. Its codebase was later spun into an open source project called Mozilla, which is the ancestor of Firefox.
- Net Applications Browser Market Share tracking (2008-06-21). Retrieved on 2008-06-21.
- Internet Explorer for Unix. - the last version of Microsoft's IE for Unix page before the product was discontinued.
- Internet Explorer 5 for Mac. - states that IE for Mac is no longer supported, and recommends Mac users migrate to [[Safari (web browser)|]]