that, which

 To understand the distinctions between that and which it is necessary to understand restrictive and nonrestrictive clauses. A nonrestrictive, or nondefining, clause is one that can be regarded as parenthetical: "The tree, which had no leaves, was a birch." The italicized words are effectively an aside and could be deleted. The real point of the sentence is that the tree was a birch; its leaflessness is incidental. A restrictive, defining clause is one that is essential to the sense of the sentence. "The tree that had no leaves was a birch." Here the leaflessness is a defining characteristic; it helps us to distinguish that tree from other trees. In correct usage that is always used to indicate restrictive clauses and which to indicate nonrestrictive ones. Restrictive clauses should never be set off with commas and nonrestrictive clauses always should.