Beginner/Novice/Adept/Intermediate/Elite/Master.

Intermediate is basically they know what they're doing and they know how to do it well. They can fight with skill and tact but would fair only somewhat well against someone who is of an elite whose honed their abilities to near master level. 

Things like versatile mean they have a lot of tricks but are a master of none of them. Unless it's their signature move for example Naruto from the series Naruto's rasengan or shadow clone jutsu. A in universe example would mean people like Aang when he uses pure air bending. He knows a lot of tricks but he's not about to kill seven fire nation soldiers like his master (presumably) did in the first season after they found the body. Aang can do a lot of things with airbending but he is by no mean feat a true master early series.

Power Benders are just that they focus on sheer force and power overwhelmingly they have an advantage on sheer stamina and ability to use copious amounts of their element and or can simply smash their way through people. They lack tactics but make up with it as stated in the aforementioned. Think of Ozai or Iroh from the original series.

Skillful benders are neither a jack of all trades or a power bender. What they know is what they got and they tend to get to extreme points such as Toph's earth bending. They specialize in one area and can do a lot in it. For an example an ice bending skill based bender could potentially stop a ship with giant blocks of ice from all sides. Or a fire bender could conjure up really precise and efficient blasts. In other words think of Mako/Azula as an example of a skillful earth bender. Water benders would be like Katara or Pakku.