From ShledonBrown.com:


IG ®
Interactive Glide is an extension of Hyperglide, in which both sides of the sprockets are contoured to improve upshifting. I.G. sprockets are very slightly thicker than Hyperglide ones, and some Shimano Hyperglide chains may hang up if used on an I.G. cassette. The slightly wider I.G. chains work on either type, as do SRAM chains.