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
I suggest this post : Best Practices & Principles for GUI design
Also, in google search, use "usability".
I also read a good book about it. But sorry, it is in French :-(
Use your users as a standard. What works best for them is what is best, so talk to them, show them prototypes and use their feedback as input to the final product.
Have a look at Jakob Nielsen's Website for a wealth of usability material.
Both Microsoft and Apple have had GUI standards for more than 20 years.
For instance expressed in the books "The Windows Interface. An Application Design Guide" (1987, ISBN 1-55615-439-9) and "Human Interface Guidelines: The Apple Desktop Interface" (1987, ISBN 0-201-17753-6). This information is also online.
The Windows User Experience Interaction Guidelines might be what you're looking for...
http://msdn.microsoft.com/en-us/library/aa511258.aspx
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.
Take a look at the User Experience Patterns (UX Patterns), by Infragistics. It's a nice source of inspiration.