essential clauses, nonessential clauses
(Slightly modified) snippets from Associated Press Stylebook 2002, typed up for my easy reference.
Both types of clauses provide additional information about a word or phrase in the sentence.
The difference between them is that the essential clause cannot be eliminated without changing the meaning of the sentence -- it so restricts the meaning of the word or phrase that its absence would lead to a substantially different interpretation of what the author meant.
The nonessential clause, however, can be eliminated without altering the basic meaning of the sentence -- it does not restrict the meaning so significantly that its absence would radically alter the author's intended meaning.
PUNCTUATION:
An essential clause must not be set off from the rest of a sentence by commas. A nonessential clause must be set off by commas.
Both types of clauses provide additional information about a word or phrase in the sentence.
The difference between them is that the essential clause cannot be eliminated without changing the meaning of the sentence -- it so restricts the meaning of the word or phrase that its absence would lead to a substantially different interpretation of what the author meant.
The nonessential clause, however, can be eliminated without altering the basic meaning of the sentence -- it does not restrict the meaning so significantly that its absence would radically alter the author's intended meaning.
PUNCTUATION:
An essential clause must not be set off from the rest of a sentence by commas. A nonessential clause must be set off by commas.