The term "pick up artist" has changed meanings and connotations so many times throughout the years that it's almost impossible to get one definition that everyone will agree with. Most guys found the community due to an experience that a lot of others can relate to. A bad breakup, hitting rock bottom, getting divorced, or even getting rejected by a fat chick can all lead you on the path toward getting good with women, but how you get there is as diverse as the people who attempt to become pick up artists. The only thing that's certain is that all pick up artists want to pick up girls.
After whatever you went through that made you want to get this aspect of your life fixed, you probably ended up at the self-help section of your nearest bookstore or poking around online. Maybe you found a few resources, and then some more, and became completely engaged. Especially back when the "PUA" movement started keyboard junkies ran rampant. Sadly, there are still some guys who read every singly blog that they can find and yet never approach or find a girl. Do not become this type of wannabe "pickup artist."
The men who don’t get stuck behind the computer and actually go out get some experience under their belts. These are probably the guys everybody thinks of when they hear "pua" - socially awkward dudes wearing ostentatious clothes asking random girls if they floss. This is not necessarily bad, but the men in this group are simply incongruent with themselves. They are still finding themselves, and on the way they are finding out who they are not.
Given time up and coming pick up artists sift through all the stuff that doesn't work for them. They give up the act for practices that are more universally attractive and proven. With experience behind them, they now have more confidence and can employ more powerful frames. After once seeing every interaction as the end of the world, they now can have more fun with push and pull and other attraction techniques. They probably consider themselves at the intermediate level and are enjoying life with their newly earned skills.
Everybody hits plateaus and obstacles throughout their learning.
The guys who keep on and expand their knowledge will become the ones that are correctly referred to as "pick up artists." The funny thing is that by that time, most of them don't even want that title. They have nothing to prove anymore, to anyone. Maybe throughout their careers they showed off their skills, but now it's innate and second nature.
There are also some less than moral pick up artist out there. Some keyboard jockeys will write false field reports for notoriety. Some "gurus" are in the community for money alone. Some philosophers will think up new theories and write them as law, when in fact they are untested. Even though these guys tend to be few and far between, they still screw up the image of the pick up artist.
The simplest definition of a pick up artist is just someone who wants to get better with women. Whether they become savvy and live a more fulfilling live is up to them. Hopefully they do not read incorrect information that leads them to a darker place than they were originally, but it does happen. A man who's successful with women has many opportunities and doors open to him that aren't available to anybody else.