Everyone starts the game at Rank 1. There are 4 class choices. Cleric, Archer, Wizard and Swordsman. Every Rank consists of 15 job levels. Upon hitting job level 15 you "Rank up" and receives a quest to change jobs. Every rank up consists of new choices, or the option to advance the current class. This is where circles come in to play.
Classes have a max of 3 circles. When you go into the 2nd and 3rd circle, in addition to unlocking a new skill or 2, you also raise the skill caps of all the previous circle skills from 5 to 10 (or 10 to 15). So a 3rd circle priest will have a skill cap of 15 on Aspersion, Monstrance, Blessing, Resurrection, 10 on Sacrament, Revive, and 5 of course on the 3rd Circle Priest skill Exorcise.
Here is an example :
I 'll just start with this pic
http://i.imgur.com/EdaUPNs.png
from a screenshot taken from this video
https://www.youtube.com/watch?v=mVmbXYvmnX8
If you didn't understand my screenshot, there's some korean terminology you want to understand first.
Korean - Romanized - Translated
서클 - seokeul - circle
랭크 - laengkeu - rank
Now if you look at the screenshot, you can see DAMAB's first 3 Rank decisions. Cleric -> Priest -> 2nd Circle Priest. The new choices are 3rd Circle Priest OR Sadhu(new) or Paladin(new).
For reference, you can check out the official character builder here
http://info.tos.nexon.com/character/...px?page=rankup
or the excellent one at tosbase here
http://www.tosbase.com/tools/class-builder/
A few minutes playing with either of these builders should help illustrate the rankings in a way that reading my boring post might not.