A "galvanized" cable is a steel cable with a rust inhibiting coating. This coating will eventually wear away (the coating corrodes first protecting the steel) and then the cable will rust. A stainless steel cable is made from stainLESS steel and thus will likely outlive the steel-reinforced housing inside of which it used. As has been mentioned, galvanized cables can be easily soldered whereas stainless cables cannot (requires special solder I believe). In the sets I have bought, Shimano ships stainless derailler cables and galvanized brake cables, presumably because a little extra friction in the brake system won't cause nearly as much of a problem as it will for the deraillers.