I have noticed that many Chinese students do not know how to properly answer this question.

Usually, when I ask this question to a new student, they will say "Sales" "HR" or "I am a businessman" or simply. "business".

First of all, "businessman" is not a job. The word business has many meanings, but saying that your job is "business" is the same as saying that your Job is a job.

"What's your job?"  "My job is a job."

A businessman is simply a man regularly employed in business. After saying you are a "Businessman" I still have no idea what you actually do. 

"Sales" is also too broad of an answer. Do you sell cars? insurance? mobile phones? Medical equipment?

"HR"? you work for a HR firm? HR in some other company? I still do not know what you do. 

Sometimes when I ask "What's your job?" I have to ask many follow up questions.

What's your job? "Business"

What business? "Sales"

What do you sell? "Phones"

              What type of phones? "Mobile phones"

              So you work for a mobile phone retailer? "No, a factory"

So here I had to ask 4 follow up questions to find out that this student is a sales agent for a factory - probably business to business, finding companies to manufacture phones for. However I am still not 100% clear.

When someone asks you "What is your job?" you need to be specific. Here is a good example:

What's your job? "I work in sales for a German die-casting plant."

or "I am a HR manager for a investment company."

          or "I am waitress at a cocktail bar."

When answering "What's your Job?" always remember to be specific. Don't make the person asking ask 4 or 5 follow up questions to find out what it is you actually do. details are important!