To put it simply,
saturation diving allows
divers to live on the bottom of the sea for a very long time. By saturating their
bloodstream with
inert gasses such as
helium, these divers eliminate their need to
decompress frequently and can withstand
extreme durations at great depths.
Since the bloodstream can only handle a certain amount of helium, by saturating themselves divers eliminate their need to surface regularly and lessen their chances of getting the bends.(I highly recommend reading Knifegirl's and ldf's descriptions of the bends to understand the terrifying consequences of decompressing incorrectly) While the bloodstream of a diver is saturated, it cannot take on any more inert gasses, and the body reaches the point of maximum capacity.
In order to keep their bodies saturated, these divers live in Deep Sea Habitats, essentially pressurized cabins on the bottom of the sea. A diver working under saturation diving conditions can reside on the ocean's bottom for months at a time, and in this day and age it is not uncommon. Currently saturation divers are employed by the Comex company to lay oil pipes across the North Sea, and a scientific Deep Sea Habitat called Aquarius exists off the shores of Florida.