As simply as it can be put..... Scale is basically "steel ash", its the waste product of combustion. As steel is heated and/or forged, a bit of the steel is consumed/burned away....how much depends on the level of heat, and the duration of exposure.
"Decarb" is a bit more complicated process overall, but can be simplified in it's terms.... What element is present in steel that can be consumed via combustion? Most prevalent is carbon. With excessive heat and longer exposure to that heat, carbon can be consumed in combustion (burnt out). This can happen a various levels. Generally it's not a major issue....all it takes is paying attention to how hot the steel gets, and how long you allow to remain at higher temps. Mild decarb doesn't give any distinct visible signs to the naked eye.... usually it manifests itself in a lower hardening of the steel, and reduced cutting ability/edge retention. In severe cases, if combustion is allowed to consume the majority of the carbon in a given piece of steel.... the steel will turn into what I call "black cottage cheese".... that's essentially what it will appear as.
There are many different methods to overcome/minimize scale.... probably the easiest for most to grasp is to remember...... leave your blades thicker/heavier before heat treat/final finishing....remember, you can always grind it off.....but if you run yourself out of steel, you can never put it back. Unless you heat steel in an inert atmosphere (no oxygen) scale IS going to happen.
The "orange peel" COULD have something to do with the heating/quenching process you use, but more often then not "orange peel" is a result of something done during finishing.... for example over buffing, or in some cases if a scotchbrite finishing product is used incorrectly/over used.