ISO just stands for "International Organization of Standardization," a body which, among other things, created standard measures for quantifying film's sensitivity to light. ISO numbers on an SLR are "equivalents" and are proportional: an ISO of 800 is twice as sensitive to light as an ISO of 400.
Also (reports my camera manual), a "high" ISO has more "noise" or "grain."
These pictures were taken with variations on ISOs to see effects on picture quality.
Below is an ISO of 200.
Below is an ISO of 400.
Below is an ISO of 800.
Below is an ISO of 1600.
Below is an ISO of 3200.
Below is an ISO of 3200.
Below is an ISO of 1600.
Below is an ISO of 800.
Below is an ISO of 200-400.
