When I tell someone that I work in movies and after all the, "Oh, did you make _________?" "Do you know anyone famous?" "How do they do (insert special effect)?" questions have passed, one of the most common questions I get is:
What does a Best Boy do?
In movie parlance, Best Boy is actually a rank, or more specifically, a pay scale. The top rank in any of the craft departments is typically called the "Key," hence the designations Key Grip, Key Makeup and Key Set PA, etc. The Best Boy is the second in command of a given department. Often though, the number two in a given department has another title because they have a particular task like Boom Operators and 1st AC's* or because custom dictates that they are called something else, like Assistant Makeup Techs. This is why only a handful of the craftspeople on set are called by this name.
Some crew members that do not carry the attendant authority are also paid on this level because they have a specific skill or position that calls for it such as Dolly Grips and FX Techs.
You are thusly enlightened.
*Camera people are covered by a different division of the Union than most other craftspeople so the ranks and payscales don't automatically match up when comparing camera to any other department. I make a point to say this because some in the industry would not consider a 1st AC and a Boom Op to be the same rank. They are probably not paid the same. However, they are both the second highest ranking technician in their departments.**
**Without getting into how the DP ranks in comparison to the rest of the crew.