ISO - What does it mean?

Since the day of film photography, ISO is always being discussed among the photographer. Until the day of digital, it starts to bring even more confusion, and always heard people talk about how the manufacturers cheating with ISO and etc.

Eventhough I heard a lot about it, I can't understand anything that everyone talk about ISO in digital age. I just know that I set my ISO on my camera, and I get the photo with desired exposure and sharpness, I'm happy. :P

It always come to my mind why there are so many variants of ISO setting for different manufacturer. Some can offer ISO 80 with no difference with the ISO 200 offered by another manufacturer, and how to know which kind of ISO offered is actually better.

Until recently, when dpreview actually post up a detail explanation on ISO, I finally see some light of the mystery of ISO...but still a bit blur blur on it.

There are 2 parts of the explanation:
1. Sense and Sensitivity Part 1
2. Sense and Sensitivity Part 2