Use commas to aid understanding and help to break up long passages of text for the reader. Refrain from over-saturating your writing with commas.
Oxford comma
Also known as a serial comma, it is an optional comma before the word and at the end of a list. Use the Oxford comma only when necessary to improve the clarity of a sentence and avoid potential confusion for your audience.
With the Oxford comma: Highlights of the tour included appearances by the President, an ancient yoga expert, and a pen collector.
is preferable to:
Without the Oxford comma: Highlights of the tour included appearances by the President, an ancient yoga expert and a pen collector.
(Unless, of course, the President really is an expert on ancient yoga and does actually collect pens.)
When writing for the web
Using commas is especially important for accessibility online, as commas affect the way screen reader software vocalises sentences to sight-impaired users.