Gecko is the open source, free software web browser layout engine used in all Mozilla-branded software and its derivatives, including later Netscape releases. Written in C++, Gecko is designed to support open Internet standards. Originally created by Netscape Communications Corporation, its development is now overseen by the Mozilla Foundation.
Gecko offers a rich programming API that makes it suitable for a wide variety of roles in Internet enabled applications, such as web browsers, content presentation and client/server . Primarily it is used by Mozilla browser derivatives, such as Mozilla Firefox, K-Meleon and Netscape, to render websites and the browser user interface (written in XUL), but it is used elsewhere as well. Gecko is cross-platform and works on a number of different operating systems, including Microsoft Windows, BSD s, GNU/Linux and Mac OS X.
Gecko is generally considered to be the second most-popular layout engine on the Web, after Trident (used by Internet Explorer for Windows since version 4), and followed by KHTML (used by Konqueror), WebCore (used by Safari), Presto (used by Opera) and Tasman (used by Internet Explorer for Mac).