WHO
Used for the person who does something, the subject of the sentence:

The man WHO bought my car.
Nobody WHO has really thought about it can possibly agree...
They caught the accountant WHO stole the money.

WHOM
For a person who is the object of the sentence. He has something done to him:

The man to WHOM I sold my car.
To WHOM did you give it?

but

In modern English, in conversation it would be more normal in the above examples to say

The man I sold my car to.
Who did you give it to? (Note that "to" has to be at the end of the phrase)

In written English, or formal situationslike a presentation, it is better to use the "correct" form:

Our benefactor, to WHOM we are infinitely grateful, has once more come to our rescue.
The directors, WHOM I know personally, are all behind me in this.
WHICH
Use it for things, object or subject.

The injection WHICH I gave him was for Polio. (subject)
The injection WHICH I recieved was for Polio. (object)
The interview, WHICH was quite long, took place in the boardroom. (subject)
I was late for the interview WHICH I had scheduled for 11 a.m.. (object)

WHOSE:
Posessive:

I spoke to a man. HIS office is near mine.
I spoke to a man WHOSE office is near mine.

The woman cancelled HER appointment. It was at 3:30.
The woman, WHOSE appointment was at 3:30, cancelled it.

Note that "whose" can be used for objects, not just people:

TMA is a company. ITS products are sold here.
TMA is a company WHOSE products are sold here.

Sometimes it is possible to use OF WHICH instead of WHOSE:

The organisation, WHOSE problems we have all heard, has little influence these days.

The organisation, the problems OF WHICH we have all heard, has little influence these days.