Isn't this polytheism? What’s the difference between Krishna and demigods? What does "demi" mean anyway?

Though we offer respect to many personalities within the cosmos, we are by no means polytheistic. Polytheism is the belief in more than one god. While there may be many controllers of various aspects of creation, neither the Vedas nor we accept there to be more than one ultimate source of all energies, more than one Absolute Truth. The Srimad-Bhagavatam names the one Absolute Truth, or Supreme Personality of Godhead, as Krishna.

Demi means "not fully" (literally, "half"). Because demigods control certain aspects of the material world, they're called "gods," with a lower case "g." In Sanskrit, demigods are called devas, godly beings. As Srila Prabhupada says in his purport to Srimad-Bhagavatam, 3.10.17, "The demigods in the higher planets are called devas because they are all devotees of Lord Vishnu."

In a nutshell; there's one God, He has many servants, and some are more highly posted than others.