«The movie which I saw last week was terrific!» «The movie that I saw last week was terrific!» You might think the sentences above are just two different ways of saying the same thing—but only one is grammatically correct. The distinction arises because words, phrases, and clauses can be restrictive (essential to the sense of the sentence) or nonrestrictive. It’s really quite simple. (Cue sadistic, maniacal laughter in background.)View The Difference Between “That” and “Which”
To copy editors, the «serial comma» refers to separation of listed elements (in a series, get it?). Specifically, the final comma in the construction «I like apples, pears, and oranges.» Somewhere along the line, newspaper editors who were obsessed with saving space decided the final comma was redundant. Today, the serial comma is omitted more often than not.View Viva la Serial Comma!