What is good English and Bad English?

You are reading too much into it. It isn't really a classification at all. When the phrase "Good English' is used it refers to the proper or correct usage of english words and phrases. "Bad English" simply means the opposite...incorrect usage, too many colloquialisms. Using "Good" english is the best way to make yourself understood by other english speakers, but nowadays people use lots of shortcuts and slang, so that what you end up with is hardly english at all.

Language is a means of trying to communicate the other person's mind through your thoughts. If we use certain words which are beyond comprehension of the other person it makes us a bad communicator. But if we use simple, understandable and unambigious words it makes us a good communicator. English as a language is not good or bad. It is we who make it good or bad. For example, take the foreign authors who have a generally high vocabulary. Now if their points are beyond our comprehension their book is of no use. Therefore they are making the language bad. But for instance we write a book with a simple English and is understood by everyone we are making the language good and also are making the other know what our points are. Therefore I believe the language is just a tool to the human beings to communicate with others. It is not an end in itself.

For me Bad English is when I can't understand what someone is trying to say in English.Not because of grammar(I'm not fluent in it and so are some native speakers :D),but of low intelligence, poor language where every second word begins with letter F.

When your talking about English, there is what you call proper English and then there is just plain English. Most people in the South uses some form of slang, Have you ever watched Jeff Foxwirthy? I don't know if I spelled his name right or not, but if you watch him you'll see how some people really does talk.