When we list a few items one by one, we separate them with a comma:

We invited Roosevelt, Washington, Lincoln.

What if the last two items are joined with the conjunction and?  Do we still need a comma? In many languages, like Russian, Spanish, Greek, German, French, the comma before the conjunction is not the norm.  In English, however, opinions among writers and editors differ on whether to use the comma between the two final items.

1) We invited Roosevelt, Washington and Lincoln.
2) We invited Roosevelt, Washington, and Lincoln.

 The final comma before the conjunction and is called a serial comma or Oxford comma.

In American English most style and punctuation guides mandate the use of this comma because it helps avoid ambiguity (confusion).

This funny cartoon demonstrates how we could understand the sentence without the final comma.  Did we invite the rhinoceri and two people?  Or did we invite two rhinoceri who have human names?