Who is a subject form. It does the action in the sentence: Who called? The caller is the subject.
Whom is an object form. It receives the action: Whom did you call? Here, the person is the object of call.
In everyday speech, many people use who almost everywhere, and that is common and natural. But whom still matters in some places, especially in formal writing and after prepositions.
- Subject: Who is coming?
- Direct object: Whom did they invite?
- After a preposition: To whom should I reply?
A quick test can help. If the answer would be he or they, use who. If the answer would be him or them, use whom. For example, Whom did you see? becomes I saw him, not I saw he.
If whom sounds too stiff in casual conversation, rewriting often works well: Who should I reply to? is common in speech, while To whom should I reply? fits a more formal tone.