James enjoys playing tennis, soccer, and basketball. (words in a series)
The troop traveled into the mountains, across the plains, and along
the river. (phrases in a series)

The car dealer made sure that the purchaser’s car was clean, that the
license plates were ready, and that the ownership papers had been
signed. (clauses in a series)

Note: If all the items in a series are joined by and, or, or nor, commas are
not required.

The chef’s exquisite dishes include filet mignon and roast beef
and lamb.

Note: If the conjunction and joins words that constitute a unit, team, or
such, do not separate that name. Yet, you will still need the commas to
separate items in a series.

Peanut butter and jelly, ham and eggs, and spaghetti and meatballs
are the children’s favorite foods.

Note: Some writers choose not to include the final comma in a series if
by leaving the comma out, the meaning is still clear.

Our social studies class members studied the Korean War, the Civil
War, World War I and World War II. (It is clear that the social
studies students studied four wars.)