Which, that and who are relative pronouns. A relative pronoun must refer to the noun or pronoun immediately preceding it (the antecedent).
 Which introduces non-essential clauses; that introduces essential clauses.
Who refers to individuals; that and which refer to a group of persons, class, type, species, or one or more things.
Whose is used to refer to both people and things.
Relative pronouns introduce modifying phrases, so you must be on the lookout for misplaced modifiers. If the meaning of the sentence is unclear, the pronoun may be next to the wrong noun/pronoun antecedent. Consider the following sentence:
 Incorrect: John was met at the door by a strange man, which he, being afraid, opened slowly.
This sentence is definitely confusing, but its meaning can be clarified by adjusting the placement of the nouns in the sentence. The relative pronoun must be right next to its antecedent:
Correct: John was met by a strange man at the door, which he opened slowly out of fear.
It's now clear what John is opening – the door, not the man. Note that the non-essential clause beginning with which is set off from the rest of the sentence with a comma.
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When working with relative pronouns, check to see that:
- The relative pronoun is right next to the word it is intended to modify
- Non-essential clauses are set off from the sentence with a comma (or pair of commas)
- Who refers to a person, not a thing; that is used for essential clauses, and which is used for non-essential clauses
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