Commas and Coordinating Conjunctions (and, but, or, nor, for, yet, so)
Use a comma before a coordinating conjunction that joins (a) independent clauses or (b) three or more grammatical items in a series.

EXAMPLE: I worked hard for seven years, and I have the promotion to prove it.

Commas and Adverbial Clauses (because, although, if, whereas)
Use a comma after an adverbial clause that precedes an independent clause; use no comma before an adverbial clause that follows an independent clause.

EXAMPLES:
Because he talked more quickly than anyone else, Adam was selected as the auctioneer.
Adam was selected as the auctioneer because he talked more quickly than anyone else.

Commas and Modifiers
OK, OK—first definitions: A nonrestrictive modifier is one that merely describes whatever it refers to; consider it as something that merely comments on the noun; it is also occasionally referred to as a "free" modifier, a phrase meaning that it can often be moved to wherever it sounds best in the sentence. A restrictive modifier, on the other hand, is one that defines whatever it refers to; it is also occasionally referred to as a "bound" modifier, a phrase meaning that it must occur in only one position for the meaning to remain constant, the same.

EXAMPLES: People who need people are the luckiest people in the world. My sister Mary carries a whisk in her purse.

The bolded expressions are restrictive modifiers; they define the words they refer to. Without them, the sentences would be either silly or unclear (in this case, I have more than one sister).

EXAMPLES: My father, Francois Maurice Berne, came to the United States from Haiti. Karen Fox, a business professor, asked me for guidelines for the use of commas.

The underlined expressions are nonrestrictive modifiers; they give further information about the words they refer to, but the sentences would be sufficiently clear without them (the meanings of the independent clauses would not change).

Notice the punctuation patterns that the example sentences provide: Use commas to set off nonrestrictive modifiers but none to set off restrictive modifiers. When the nonrestrictive modifier occurs in midsentence, use the same punctuation mark on both sides. To give less emphasis, you could use parentheses in those locations; to give greater emphasis, you could use dashes.