Check out the relevant Wikipedia page, for starters. The Divine Right of Kings as a notion went back to medieval times, when rulers would only rule with the blessing of the Pope, though the theory so named was not spelled out until later.

I'm not very impressed with the Wikipedia article on the subject. It seems to conflate several different theories of civil authority that were historically used in opposition to each other. According to the Catholic Encyclopedia, The Divine Right of Kings asserts that:

...in a State once monarchical, monarchy is forever the only lawful government, and all authority is vested in the monarch, to be communicated by him, to such as he may select for the time being to share his power. This "divine right of kings" (very different from the doctrine that all authority, whether of king or of republic, is from God), has never been sanctioned by the Catholic Church.

If I'm not mistaken, The Mandate from Heaven and medieval divine right theories were generally thought to limit a kings power (by making it subject to revocation by religious authority), whereas The Divine Right of Kings (the Post-Reformation concept; note the capitalization) seeks to expand monarchical power. In any case, it would be wise not to update very strongly on my vague impressions from my university days (which are somewhat hazy and long before I started using spaced repetition software).