(interrogative) What person or people; which person or people.
(relative) Used to refer to a previously mentioned person or people.
That is the woman whom I spoke to earlier. (defining)
Mr Smith, whom we all know well, will be giving the speech. (non-defining)
He's a person with whom I work (defining)
We have ten employees, half of whom are carpenters. (non-defining)
There is a new girl in the class whom I consider to be a wunderkind.
She felt miserable unless she had neighbors with whom to quarrel regularly.
(fused relative, archaic outside set patterns) The person(s) whom; whomever.
To whom it may concern, all business of John Smith Ltd. has now been transferred to Floggitt & Runne.
(informal, especially non-US) Also used with names of collective nouns that are groups of people, especially singularly-named musical groups or sports teams.