Used primarily for one of three purposes: 1. to create compound sentences (with a conjunction), 2. to set off dependent or subordinate clauses, 3. to separate lists.
Example: Keeping it between you, me, and Marcia, I want to quit my job, and I want to start a pogo-stick making factory. [the commas between “you, me, and Marcia” create a list; the comma between “Marcia” and “I” separate a dependent clause from an independent clause; and the comma between “job” and “and” creates a compound sentence.]