The basic rule
Who is a subject pronoun, used when the person is doing the action, like "he" or "she": "Who called you?" (He called you.) Whom is an object pronoun, used when the person is receiving the action, like "him" or "her": "Whom did you call?" (You called him.)
The he/him trick
Try answering the question with "he" or "him." If the answer is "he," use who. If the answer is "him," use whom (both end in M — who-M, hi-M).
Examples
Who: "Who is coming to the party?" "Who wrote this book?"
Whom: "To whom it may concern." "Whom should I ask for help?"
A note on modern usage
In casual, everyday speech, many English speakers use "who" for both cases ("Who did you call?"). "Whom" is still expected in formal writing and after prepositions ("to whom," "for whom," "with whom").
Frequently asked questions
What's the fastest trick to choose who or whom?
Answer the question with "he" or "him." "He" → use "who." "Him" → use "whom" (both end in -m).
Is it OK to just use "who" all the time in casual speech?
Yes, informally it's widely accepted, though "whom" is still preferred in formal or written English, especially after prepositions.
"To whom it may concern" — why whom here?
Because it follows the preposition "to," which always takes the object form ("whom"), not "who."
Is whom disappearing from English?
It's becoming rarer in casual speech, but it's still standard in formal writing, legal documents, and careful prose.