Info on Gecko | XulRunner

In short:-

Gecko is Soul.

XulRunner is Body.

In Brief:-

Gecko is the name of the layout engine developed by the Mozilla Project. It was originally named NGLayout. Gecko's function is to read web content, such as HTML, CSS, XUL, JavaScript, and render it on the user's screen or print it. In XUL-based applications Gecko is used to render the application's user interface as well.

Gecko is used in many applications, including a few browsers, such as Firefox, SeaMonkey, etc. (for a complete list, please refer to Wikipedia's article on Gecko.)

XULRunner is a Mozilla runtime package that can be used to bootstrap XUL+XPCOMapplications that are as rich as Firefox and Thunderbird. It provides mechanisms for installing, upgrading, and uninstalling these applications. XULRunner also provideslibxul, a solution which allows the embedding of Mozilla technologies in other projects and products.