"Who" has something in common with Latin and German nouns. They all change form depending on their function in a sentence just like verbs change form depending on their tense. This system of changing nouns is called a case system.
Like Latin, German, and many other languages, Old English had a case system. Modern English still has a few words that change their form, but won't change their ways: "who" and "whom."
Because this form change is unusual in modern English, many speakers don't know when to use "who" and when to use "whom." Don't worry, we're here to help!
Put technically, use "who" as the subject of a verb. Use "whom" as the verb's object.
Put simply, compare "who" and "whom" to a more familiar pronoun.
who = he
whom = him
When in doubt about whether to use "who" or 'whom," ask a question, and answer it with "he" or "him." Now, who wants some examples? We bet he wants some examples.
"Who threw that squirrel?" "Who" is the subject of the verb, and "He threw that squirrel" sounds better than "Him threw that squirrel." If we were being honest, however, it was us. We threw that squirrel.
"At whom did we throw that squirrel?" "Whom" is the verb's object, so we'll confess by saying we threw that squirrel at him. We most certainly did not throw that squirrel at he. That would be wrong.
People rarely confuse "he" and "him." With a little practice, "who" and "whom" will be just as easy to use.