‘That’ is used to describe objects and non-human things. ‘That’ is used to introduce essential information and requires no additional punctuation or comma, i.e in a defining relative clause. While ‘Which’ is used to introduce non-essential information, commonly separated by commas i.e a non-defining clause. For example, The telephone, which was ringing, was in the living room. The cow that is very sick needs to see the vet today.