Well the one weight simply refers to how much the servo in itself actually weighs when you drop it on a scale. Most standard servos for instance are around 50grams each more or less. A sub micro servo would be in the 10g or less area. You get the idea.
Then the other weight refers to the amount of torque the servo is. Literally how strong it is. I'm going to be lying when I try to explain how they actually measure it I'm afraid, hopefully someone will shed some light on that for both your and my sakes. But regardless, we know for instance that most standard standard servos that comes with radio packages and are used by most guys are 3,1kg/60' for instance. These servos are usually good for .40 sized aircraft like trainers and and. A 5kg/60' servo would be a nice upgrade for these types of aircraft and have a little more punch to move the control surfaces. On our big .90 size helicopters we like to use anything from 9kg~15kg torque ratings. On big giant scale aircraft guys will put for instance two at least 15kg servos on an aileron on big 33% aircraft but it would all depend on the aircraft's design of course and the size of the control surfaces.
