That - is used when the information is essential(must be included)
- She prefers to rescue dogs that are special needs (sick, injured, old etc).
- Do you have all the food that you need for the party?
- She wanted to have a house that is big enough for the whole family.
- My sister likes to eat food that is health.
- I want to live in a place that is close to work.
Which - is used when the information is not essential
- We like to watch American movies, which are in English.
- I have to clean up my room, which is a mess, before I go to the movies.
- The English test, which I did not like, was very hard.
- I bought a new car, which is red, was really expensive.
- See more at: http://www.english-the-easy-way.com/Confusing_English/What_Which.html#sthash.MTOq6Kaf.dpuf