As the name suggests, the headset is integrated in the frame of the bike. The bearings are directly placed into the frame. This means that there are no cups between the frame and the bearings. This type of headset gives you the code IS.
Zero Stack (ZS)
Zero Stack, or also known as 'Integral' or 'semi-integrated' means that the bearings are inside the frame tube as well, but the difference with Integrated is that Zero Stack type headsets have a separate cup between the bearing and the frame (as you see in the illustration, there are 4 parts in both upper and lower part of the headset instead of 3 as in the illustration of the Integrated headsets). This type of headset gives you the code ZS.
External Cup (EC)
The External Cup type is most obvious as you can see the bearings above and below the frame tube. The bearings are not exposed but hidden in a cup that slides into the frame. This type of headset gives you the code EC.