Human User Interface guidelines
Human User Interface guidelines are a list of rules that applications should adhere to, in order to share a common look and feel with other applications in a particular desktop environment.
If all applications look, feel and behave in a similar manner across a desktop environment, a user can transfer skills learned in one application to another. This will make the user more efficiant, and make learning to use new programs easier. Human User Interface guidelines will dictate a set of rules for general usability, as well as visual design including icons design and style, user input and interaction and language style.