Pantone Matching System, I believe. Access them from the swatches palette, by clicking on the Options button, and going to "Open Swatch Library."
As far as what they are...it's a universally accepted set of spot inks. You can count on these colors to look the same no matter what offset printer they are printed on or what printer you go with. As opposed to simple CMYK (process) where one mix of ink would not necessarily be identical to another mix of the same percentages.