Global sources are sources that are independent of scenes, and persist until the stream ends. They start up when first used, and are destroyed only when a stream ends. A regular source on the other hand is created when the scene is created, and destroyed when the scene changes. For a web camera and capture cards you do not want this to happen as loading and unloading devices takes time, so global sources are especially useful and recommended for such video devices. Bitmaps and similar sources on the other hand do not need to be global sources as loading and unloading them is near instant.
Note: When you modify the properties of a global source using the sources list box in the main window, it will affect all scenes the global source is added to.
First, create a new global source by clicking "Add" in the global sources window.
Setup all necessary settings and click "Ok" to add it to the list of global sources.
To then use your global source in a scene, simply right click the sources list box, select "Add" then "Global Source", then choose the global source you wish to add to the scene.