Use commas to separate three or more words, phrases, or clauses in a series.
A conjunction goes between the last two items of the series.
While some authorities say that the comma before the conjunction is optional, leaving it out may cause confusion, so it is better to include it.
Words: Use commas to separate three or more words, phrases, or clauses.
Phrases: This morning I woke up, got dressed, brushed my teeth, and ate breakfast.
Clauses: In fact, the bus was full of people who got dressed, who brushed their teeth, and who ate breakfast this morning.
Incorrect: The street was filled with angry protestors, shouting spectators and police.
(Leaving out the last comma makes it look like the police were shouting, too.)
Correct: The street was filled with angry protestors, shouting spectators, and police.
(Makes it clearer.)
See also Semicolons in a Series.