what are different groups for stocks in trade market?

#1
Hi everybody,
I am new to trading and trying to understand its nature before actually going seriously into it. :p
I have seen that all the stocks are grouped into various groups like 'A' , 'B' .....etc. I want to know that what is the significance of these groups? Is it safe to trade in group 'A' stocks? What is the criteria for these stocks for lying under a specific group?
And does it play any role in stock selection?

Thanks in advance.