Use commas to separate three or more items in a series.
(A conjunction goes between the last two items in the series.)

Words in a Series

Commas can separate words, phrases, or clauses.
I like to eat tacos, pizza, and steak.
Eileen is tall, thin, and attractive.

Phrases in a Series

I rode the bike, walked on the treadmill, and lifted weights.
Katie likes to play cards, sing songs, and paint flowers.
Elizabeth eats quickly, cleans up sloppily, and leaves stealthily.

Move on to Rule 5.