To this Minnesotan in the middle of winter, pretty much what it says.
A "slick" surface is smooth, and will tend be slippery (though a slick smooth floor is not slippery to rubber-soled shoes, for example). Ice is usually but not always "slick".
A "slippery" surface is one which is apt to result in slipping (and possibly falling, if walking across it). The slipperiness could be due to ice, oil, loose gravel, etc.
(Though admittedly the terms are often interchanged with little regard, especially by radio announcers looking for a different way to describe road conditions.)