It depends on the usage as a conjunction they are almost equal ex: i loved the sandwich that i ate. i loved the sandwich which i ate Which can be used as a question word : which of you have seen my keys? but that can not be used as a question word that can be used a pointing word : that boy/girl is cute but which can not be used as a pointing word

"that" is used in defining relative clauses: I don't like serious music, I like music that makes me want to dance "which" is used in non-defining relative clauses: This food, which I really love, is typical of my country Nowadays, however, "which" can be used instead of "that". However, "that" cannot be used for "which". For example: I don't like serious music, I like music that makes me want to dance (correct) I don't like serious music, I like music which makes me want to dance (correct) This food, which I really love, is typical of my country (correct) This food, that I really love, is typical of my country (incorrect) If you want to find out more, Google "defining and non-defining relative clause"