ISO setting - what it really means ?

More newbie questions.

I am just wondering what the ISO rating in a digital camera really means.
On film it is the light sensitivity of the film itself. But for digital
camera the light sensitivity of the CCD-sensor does not change. What does
change when the ISO setting is altered ?

Another question : does the ISO rating of different digital cameras match
one another - i.e., correspond to the same light sensitivity ?

Also, does the ISO setting of digital cameras matches with the ISO number
of the film ? That is, if in a particular situation for ISO 100 on a film
camera, the correct exposure setting is 60/F4, will that be true for a
digital camera set at ISO 100 as well (using the same lens) ?

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Gautam Majumdar

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