That vs. Which

That is used with essential clauses and does not require commas. In other words, what "that" is describing cannot be removed, or the sentence would not make sense. "That" is used with things or groups; "who" is used to describe people.

• He goes to a school that has the highest academic standards.
• I love movies that are insightful but end in a positive way.
• She is a person who has strong political aspirations.

("He goes to school," does not convey the complete meaning of the first sentence; "I love movies," is not the point of the second sentence. In the third sentence, "who" is used because we are describing a person not a thing or group.)

Note: When a sentence contains this, that, those, or these you may use which to introduce the next clause. This is true with both essential and non-essential clauses.

• This is a topic which I am unable to discuss.
• Those are the earrings which I will wear to the party.

Although both of these sentences are grammatically correct; both could be reworded for a bit more clarity:

• I am unable to discuss this topic.
• I will wear those earrings to the party.

Which is used with non-essential clauses and is usually surrounded by commas. In other words if the clause was removed it wouldn't change the sentence.

• She had a dog, which was incredibly smart, that she brought to the competition.

• Her opinion, which was always influenced by her mother, was to go ahead with the remodel.

(In both of these sentences if you take out the clause between the commas the sentence would still make sense: She had a dog that she brought to the competition; Her opinion was to go ahead with the remodel. I would also argue that the first sentence, although grammatically correct, would sound better: She had an incredibly smart dog that she brought to the competition.)