Once again, I would like to hear your opinions.
This time, it is "," ... yes, a comma.

(A) She married a guy whom she met on the internet.

(B) She married a guy, whom she met on the internet.

... as you can see, the only difference between these two sentence is No.2 showing a "comma" in front of the relative pronoun "whom".

1. What are differences in meanings of (A) and (B), if there is any?
2. Are these must be precisely differentiated in real-life usage?

I understand that one single comma could make a sentence have different meanings in certain cases.

However, I would like know the effect of comma in these particular sentences, (A) and (B) above.

Thanks in advance.
Excellent, Anon! Emotion: smile
In this case you have restrictive (or defining) and non-restrictive (non-defining) relative clauses.  
Non-restrictive clauses use commas; restrictive clauses do not use commas.

Non-restrictive relative clauses give extra information about the noun they are describing; restrictive clauses give us defining information about the noun being described.  
It helps to look at the following examples:

Pirates who kill people go to jail. (restrictive)

Pirates, who kill people, go to jail. (non-restrictive)

In the second sentence, 'who kill people' is extra information about pirates.  We can take out the relative clause and it doesn't change the meaning.  If we take out the relative clause, we get 'pirates go to jail'.  This means all pirates go to jail.  It also means that all pirates kill people.

In the first sentence, 'who kill people' is important information.  The first sentence, then, says that only pirates who kill people go to jail.  Other pirates do not.

Your example is not a particularly good one because we actually need the relative clause to tell us which guy she married.  It should only be restrictive.  In other words, your example with commas is probably wrong.

When talking about things (instead of people), it is important to remember that 'that' can only be used in restrictive clauses, and that 'which' can be used in either restrictive or non-restrictive clauses.
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 Cool Breeze's reply was promoted to an answer.
Well said and excellent examples.
I have no more questions on this topic.

Anon, you are the man/woman, and I love you.

Again, thanks a lot.