Gecko is used in multiple browsers, including Mozilla Firefox, SeaMonkey, Camino, and others. Gecko is continuously under development at mozilla.org. Gecko has been known previously by the code names "Raptor" and "NGLayout"; the new name was chosen following a trademark infringement dispute.
Gecko is so fast and so powerful that it's being used to create the user interface of some applications ("chrome") as well. In other words, Gecko will not only be displaying the document's content, but it will also be painting the scrollbars, toolbars, and menus on the screen as well.
Gecko is written in C++ and is cross-platform, running on operating systems including Microsoft Windows, BSDs, Linux, Mac OS X, Solaris, OS/2, AIX and OpenVMS. Its development is now overseen by the Mozilla Foundation. Licensed a tri-license of the Mozilla Public License (MPL), GNU General Public License (GPL) and GNU Lesser General Public License (LGPL), Gecko is free and open source software.
Gecko is the second most-popular layout engine on the World Wide Web, after Trident (used by Internet Explorer for Windows since version 4), and followed by WebKit (used by Safari & Google Chrome) and Presto (used by Opera).
Gecko includes the following components:
• Document parser (handles HTML and XML)
• Layout engine with content model
• Style system (handles CSS, etc.)
• Image library
• Networking library (Necko)
• Platform-specific graphics rendering and widget sets for Win32, X, and Mac
• User preferences library
• Mozilla Plug-in API (NPAPI) to support the Navigator plug-in interface
• Open Java Interface (OJI), with Sun Java 1.2 JVM
• RDF back end
• Font library
• Security library (NSS)