Whom.

Your question should be, "What's the difference between 'Whom' and 'To whom'. Anyway, 'Whom' is used to ask an indirect object of a verb and 'To whom' is used to ask an object with a preposition. For examples,
Whom do you love?
To whom are you talking?

Sorry, I missed a question mark.

frnd,i mean i can not make a question by 'to whom'instead of 'whom'?

viz,whom always used for statement,where as 'to whom'used for interro gative?

No. See tenjing's answer above.
Whom did you invite?
To whom did you send the invitation?

As a relative pronoun, whom can be a grammatical object in a relative clause while the 'to' in to whom occurs because something else requires it. Examples:

He is the man [whom/who/that] I saw there.
This is Mr Smith, whom you haven't met.
This is Mr Smith, with whom I came here.

This is Mr Smith,

The difference between 'who' and 'whom', is exactly the same difference as between 'he' and 'him'.

I heard we don't use 'that' for people.

Cool Breeze:

As a relative pronoun, whom can be a grammatical object in a relative clause while the 'to' in to whom occurs because something else requires it. Examples:

fatimah0786I heard we don't use 'that' for people.

In some contexts, such as CB's example sentence, you can use that for people.

fatimah0786I heard we don't use 'that' for people.

That is the oldest relative pronoun in English and it was used to refer to people for about 400 years before who as a relative existed. It is, of course, only possible in defining / restrictive relative clauses.

CB