It's to whom, actually...
There's truth in this terrible joke. Despite not being commonly used, "whom" is not only a word, but a word that should be used. More often than not, the word who is used in place of whom. Today we'll be explaining the correct (and therefore best) way to use these troublesome words.
Who is a pronoun and the subject of a verb.
E.g. "Who brought all the beers?"
We can't say it much simpler than that.
Whom is almost identical to who, with the main exception that it is the object of the verb.
E.g. "He brought all the beers for whom?"
If you recall all the word categories then this shouldn't pose a problem. If you can replace the word with "I", "he", "she", "we" or "they", then you should be using the word "who". If you can replace it with "me", "him", "her", "us" or "them", then "whom" should definitely be used.
Of course there are very few people who still observe the rule and even fewer who are bothered by its misuse. If you are losing sleep over it, now you know!