there have been many excellent posts on this subject, the first being ray zee's post followed by many others (such as aejones/etc). here is something i've observed when focusing on my own play and others and the developments that occur.
a player first starts fairly poorly, making fundamental mistakes. these mistakes can be categorized in many different ways, namely either being too tight, too loose, being too passive or too aggressive. many poker 'style's are predicated from one's personality (and obviously the better players adjust to this, more on that later). the beginning player is often making these fundamental errors as well as dynamic ones, not only limping J4s UTG but also open shoving 150bbs into a 8bb pot on the flop. many players obviously do not make it past this first stage, in which their are so many errors in terms of overall play that losing is inevitable.
the second stage (imo) is the first start of the learning process. be it for one reason or another (forums, books, chatting w/ better players), the player 'evolves', and begins learning actual strategy. this player realizes that 60/20 is not the way to go. or 19/8. he starts to start thinking about things a little. now, this player may still make some fundamental mistakes and logic such as raise/folding an overpair for 'information, or to see where i was at', but that's ok, at least for now. he stops making the horrible mistakes he made in the past, that is, to limp/call Q9 UTG and such. he stops calling 24s OOP to a PFR. he starts to make better decisions, stops slowplaying as often. this player starts to think.
further, as players get better, the learning process continues. the fundamentals get crisper. they start to learn pot odds, range weighting and other fun math. these players know to start raising more in position and to stop playing so much out of position. they start grasping other simple maneuvers such as check raising as a bluff and semi bluff, to start raising c-bets in position, to start floating. many players get to this stage, robotic tagbots that play fairly straightfoward and take money from fish while breaking even vs. regulars. many players inhabit the micro and ssnl limits.
from here, players build on what they know. they start to balance their ranges. they start to manipulate perceived ranges, both from them and their opponents. they start to get into 'leveling wars'. they learn to make tough folds, they learn to stop making stupid plays where their lines make no sense. they start to hand read, they start to think a few streets ahead, and their reraising ranges are not only premiums. they know when to make the right plays depending on opponents and are not balancing their ranges vs. fish. alot of these players are seen in small to mid stakes games.
from here, i can only guess. the HSNL world of poker are several universes ahead of me, and i can't even comprehend some of their thought processes. even though poker is a seemingly simple game, there are many many levels in terms of mathematics, game theory, psychology, etc. that are involved. it's funny to think about it on a grand scale...players do very weird unconventional things when they first start, when they are fish...as they get better, they go from typically a very tight process to a lag/tag one, and finally when they reach the highest stakes, they revert back to unconvetional mode...in that they do many unconventional, non standard things, just like when they were fish. they make plays that don't always make alot of sense, because it keeps them one step ahead of their competition. there are still some that can succeed by playing good, solid, fundamental poker (leatherass, etc). but the very best play differently (durrr, ivey, etc).
poker styles are not unified either. players may adopt one style for most of their career (be it tighter tag, lag/tag, lag, etc) but learn to adjust. they adjust based on the dynamics given, the history and opponent type. no longer do they have only one style, but multiple.
anddd i could be wrong on all of this. but i think its accurate in alot of ways.
