That is used when the information being added is important to the sentence.
- The dog that is walking down the street is very big.
(that is walking down the street is important because without it we wouldn’t know which dog it is.)
Which is used when the information being added is not important to the sentence. If you use which you also need to use commas.
- My kitten, which was very friendly, ran away yesterday.
(Without which was very friendly the sentence still means the same.)
My kitten ran away yesterday.