One of the richest men in the world is the appositive.
It has a comma that separates it from the rest of the sentence and the sentence makes sense without it as "Bill Gates started Microsoft."
If Bill Gates had a comma after it, it would be the apositive and the sentence would be "One of the richest men in the world started Microsoft," but since there is only one comma the appositve is "One of the richest men in the world."