A level pool players
These players are at the top of the local amateur players. In local tournaments, they often finish in the money. They have strong players with good offensive and defensive skills. Generally, they know every one in the area who is at their level. Generally, an unofficial skill ladder exists. Oftentimes there will be some complicated handicapping negotiations before they play with each other. Because of the small number at this level, these handicaps change on every competition based on who won on the previous match. These negotiations can be entertaining by themselves.
These players are the regulars who have a strong interest the the game. Pool will usually be their first sport of choice. They play at least once a week (sometimes more) and are willing to put time in on the practice table. B players are reasonably competent shooters. They enjoy participating in team leagues and local tournaments. If they can place in the money once in a while, they are thrilled and encouraged to continue.
They are decent competitors who love the game and its competitive, yet friendly, environment. They'll play safeties once in a while - usually when the table layout is extremely poor. They will attempt many shots that are on the edge of their comfort zones.
Players at the C level are at the largest population of shooters across the planet. They play regularly (couple times a month or more) and are acceptable shooters. They are willing to learn (sometimes) and have a reasonable grasp of some of the ins and outs of the game. They don't play much defense - and leave opponents with bad layouts, usually accidentally.
If a better shooter offers suggestions and recommendations, they will (usually) pay attention. They are just learning their stance and stroke. If they are lucky, they will be in the company of more skillful friends who can help them through their basic learning curve. C players enjoy playing. Pool playing will be a part of their life, but there won't be a the passionate interest level that will force them to the practice table for any consistency.
Individuals who pick up a cue stick maybe once a year gain the "D" level designation. Sometime when they were first introduced to pool, they may have been enthusiastic for a short time - but quit, either through frustration, or because some other interest. These people are perfectly happy picking up a stick and banging balls - but are doing so only for fun, not any real interest in figuring out how to be better. They are most obvious because of the numerous violations of fundamentals - they'll have weird stances, strange bridges, chicken wing strokes, etc.
Sometimes this simple scale is extended into three sub-levels. This gets nine skill levels: C-, C, C+, B-, B, B+, and A-, A, A+. The ratings are most often seen in small local tournaments. Determining these levels are at the discretion of the tournament director. And, of course, this opens the opportunity for the TD to play favorites. No amount of complaining or protesting will do any good in adjustments, since these TDs usually have hearts of stone. Generally, if you win one of these tournaments, you will find yourself being raised up for the next tournament.
Keep in mind that these ratings are arbitrary and pretty much in the eye of the beholder. They are best used among your friends when determining handicaps when you all get together to play.