So-called American "cheese" is a processed food that contains only a minimal amount of actual cheese.
Relative pronouns begin relative clauses. If the relative clause is a "restrictive clause" defining the noun that precedes it, the clause is not set off by commas and "that" is used: "The one that you want is over there" (in this example, "that you want" is the relative clause). Restrictive clauses are essential definitions; often if you were to omit the relative clause the sentence would make no sense. Sometimes you can cross out a restrictive clause and still end up with a reasonable sounding sentence (as for "The book that you want is over there"), but the meaning is not clear without the definition provided by the restrictive clause.
A non-restrictive relative clause, on the other hand, is not essential to the meaning of the sentence. It gives additional information, but the meaning of the rest of the sentence wouldn't change if it were to be omitted. It is typically set off from the rest of the sentence by commas: "My favorite book, which I re-read regularly, is Strunk's `Elements of Style.'" If you can put the relative clause in parentheses without changing the basic meaning of the sentence, it is a non-restrictive clause. Non-restrictive clauses begin with "which" (when they refer to things, that is -- if the relative clause refers to people, there are different relative pronouns to worry about, but I won't get into that). Note that a comma should precede the use of "which."
Personally I think that when to use which is the most subtle rule of English grammar, particularly since I didn't learn it until my 20s, and therefore I may not always catch a misused "which" when I check your papers. But for readers who have internalized the rule, misuse diminishes the clarity of your writing. So if you haven't internalized the rule, make a special effort to consciously double-check that every "which" in your paper is really introducing a non-essential description, not an essential definition.
Bryan Catanzaro wrote this useful script for detecting potential misuses of that and which.
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