It is my understanding that when dealing with distances, a unit of 1 equals approximately 1 meter. When dealing with Rigidbodies what are the units of the following:
velocity - components of the vector. Are they meters /sec, meters per hour, etc.?
drag - What are these units in and is the scale linear? For example, what does it mean when I set the drag to 10?
mass - Since the units need to be between around 0.1 and 10, with being closer to 0.1 better, is it just a relative setting? Ex. Does an object with a mass of 1 have 10x the mass of an object with a mass of 0.1? Also, how does this play into applying forces? I assume the mass of an object might impact how effective applying a given force is.
AddForce() - In what units am I applying force?
Neil has inspired me so I am playing around with applying forces to rigidbodies and would sorta like to know what the units are in so I do not feel like I am completely hacking everything 
It would be nice to know but do you really need to? I always just tweak. I didnt think that physics engines could really match real world physics making real world units kinda usless. Also kinda hard to match real world units when things like the max mass value is 20. Obviously in the real world you could have a mass of any thing. Those are just physics engines limitations.
Bill
I find it helpful to at least know if they are exponential, linear, and the like, and if the relationship can be explained at a medium level. I am not looking for formulas, just rules of thumb. Although I tweak, I like to have some idea where I might want to start from. For example, if I am creating something that flies, I might want to start out with a drag and an angular drag of 10 so I won’t be flying sideways after I turn. I assume the physics engine uses this information somehow.
This all started with trying to control my object and wondering if the problem was my script that took input and applied forces or some setting that I needed to change. I might be getting to detailed, but if it is readily available it might be helpful when starting out and to help when debugging (ex. “Why isn’t my object moving, too little force or my script is not applying the force correctly”). I do feel like I am starting to get a “feel” for drag but I could not explain how to use it at all besides just saying play with it.