Classic...The Classic design headset fits in a traditionally designed head tube and utilizes external bearing cups. There exist long established standards for head tube and fork interface dimensions with this design. Historically there have been two primary sizes, 1" and 1-1/8", for the two most common fork steerer tube outer diameters.
The 1-1/8" size, historically the standard for mountain bikes, has become the standard for road and BMX bikes as well.

Zero Stack...The Zero Stack (ZS) design headset requires a relatively large head tube and utilizes bearing cups that are hidden or partially hidden inside the head tube. The Zero Stack headset offers a low stack height, a clean look and the security of bearing cups.

Cane Creek IS System...The IS System (IS) design utilizes no bearing cups and requires a carefully machined head tube to specific bearing interface dimensions. The Integrated System design is extremely dependant upon frame quality and finishing, and is very sensitive to frame, fork and stem tolerances.