What is the difference between that and which?

The difference between that and what depends on how they are used. One example: "That boy over there is very tall". "That" is in reference to a specific boy, the tall one. "That" is usually used to specify a particular person place or thing (noun). "What boy?" When using "What" you are not specifying a particular boy, but asking which boy are we specifying. "What" is usually used to ask a question. You may be asking the question due to hearing "what" used improperly in a sentence like "He is not the boy what I saw." In this case "what" was used in place of the proper choice of "that".