Gecko is a web browser engine developed by mozilla (mozilla is the Mozilla Foundation and the Mozilla Corporation). Gecko is open source, and used in Mozilla Firefox and Mozilla Thunderbird etc. It's designed for the open internet standards and (as a web engine does) used to render web pages. Gecko is also used for a UI in a application (used with XUL). Gecko is written in the programming language C++ and is compatible with various operating systems. The development is seen under the Mozilla Foundation and is licensed under the Mozilla Public License (version 2).