If you want the dictionary definition, just double click on any word.
Who and whom are both pronouns.
Who is used to give further information about a person or people previously mentioned in a sentence.
Basically anytime "I", "she" or "he" 'feels' right, who can be used.
Lynne built that funny English website.
She is the person who built that funny English website.
In questions who is used when asking which person or people did something, or when asking what someone's name is.
"Who is that woman over there?" - She is the boss.
"Who let the dogs out?" - I didn't do it, he did!
"She asked me if I knew who had got the job. I had to tell her she hadn't got it."
To be honest, in informal writing and speech who is used most of the time, and poor old whom seems to be on its way out. In fact Theodore M. Bernstein, a former New York Times editor went so far as to campaign to have whom banished from the English language altogether, saying, "I favor whom’s doom except after a preposition."
However, if you are taking an English test or sitting an exam, it's best to know when and where to use it.
Whom is used in formal English instead of who as the object of a verb or preposition.
"For whom the bell tolls."
"There were 500 passengers, of whom 121 drowned."
!Note - Whom is rarely used in questions. For example:
"To whom do you wish to speak?" (This sounds very old-fashioned and stilted. You are more likely to hear "Who do you want to speak to?"
!Note - If in doubt, try the he or him test:-
Try rewriting the sentence using he or him .
"He took out a photo of his son, whom he adores." - "He adores he" should 'feel"'wrong. So it must be "He adores him."
If you're still not sure and it's not an exam or test, go with who, 99% of the time you'll be right.
!Update - I have had to take quite a bit of grief about this post, usually from English teachers who don't want to stop telling their students to say "Whom did he marry" etc. So, if you don't believe me, perhaps you will believe McMillan education:-