As a demonstrative pronoun (pl. Those), that usually points out, or refers to, a person or thing previously mentioned, or supposed to be understood. That, as a demonstrative, may precede the noun to which it refers; as, that which he has said is true; those in the basket are good apples.
As an adjective, that has the same demonstrative force as the pronoun, but is followed by a noun.
As a relative pronoun, that is equivalent to who or which, serving to point out, and make definite, a person or thing spoken of, or alluded to, before, and may be either singular or plural.
As a conjunction, that retains much of its force as a demonstrative pronoun.
As adverb: To such a degree; so; as, he was that frightened he could say nothing.
OR which -- "that" if the information is essential to understanding the point of the sentence (it restricts the scope of the noun) -- "which" if the information provides additional but nonessential detail in the sense of "by the way"; put a comma before "which" Here's an illustration that might make this clearer. Let's say we're looking at a bunch of piglets. I want to direct your attention to a specific piglet. I might say, "Look at the piglet that has brown spots." In this case, "brown spots" helps you find the piglet I'm talking about -- it restricts the noun (piglet). Next I might say, "This bin, which I got at a farm sale, lets all the piglets eat together." In this case, it doesn't really matter where I got the bin -- that information is not necessary to understanding that the piglets enjoy communal dining.