GUIs are one of the important areas in most of the software products. I would like to know whether there is something called GUI Standards that defines the best practices in GUI
Depends what you're doing. Nearly every platform has their own official and usually a few unofficial ones.
That's the beauty of standards if you don't like it pick another one
. However GUI is not easy to standardise as there are multiple IO mechanisms - visual, non visual, one way, interactive etc.
No one standard could fit all - except perhaps and ISO or another standards body and that as we know will be obsolete before it's even completed a first draft.
On the other hand there are various psychologically based user experience techniques that are slowly becoming de rigeur standards.