Water containing less than 17.1 parts per million (1 GPG - or grains per gallon) of salts, calcium, magnesium or lime that may interfere with the formation of suds and lather with soap. Because of this, soft water feels slimier than hell, and it if you bathe in it, you could be there for about an hour trying to get the soap and shampoo off of yourself. It's very unpleasant if you are not used to it.
Soft water has gone through a softening and purifying process where the minerals are extracted from the water - a side effect of which is an increase in the amount of sodium. People who use water softeners should consider taking mineral supplements, because the body requires both calcium and magnesium.
Soft water in and of itself isn't any less potable than hard water (although its blander), however drinking soft water from the tap is probably not a good idea. Water is a universal solvent and the purer the water is, the more aggressive it is in leaching your pipes of lead and copper. These metals are generally considered to be unhealthy for human ingestion.