Novice Job ClassAll characters in Ragnarok Online start off as a Novice. After obtaining level 9 of the Basic Skills skill, players may change their profession to a first job class. The one exception to this rule is the Supernovice class, which additionally requires that the character be at least base level 45.
First Job ClassesFirst job class characters have specialized skills that are specific to the job they choose. Currently, there are eight first job classes available in iRO. The Gunslinger and Ninja job classes have been implemented into iRO Sakray as of January 2, 2007 at 9:00 PM PST during the weekly scheduled maintenance. The maximum job level for first job class characters is 50, except for Gunslinger and Ninja for which it is 70 and Supernovice for which it is 99.
Second Job ClassesAfter obtaining job level 40 in a first job class, a player may job change to a second job class that his or her job class is eligible for. The three exceptions to this are the Supernovice, Gunslinger and Ninja job classes, which are not eligible for any second job classes. The maximum job level for second job classes, like most first job classes, is 50.
Transcendent Job ClassesAfter reaching base level 99, job level 50 in a second job class, a player may transcend, where he or she will be reborn as a level 1 Novice. The exceptions are Taekwon Master and Soul Linker, which are not eligible for transcendence. By transcending, he or she will have to level up from the beginning once again, but when he or she changes to his or her second job class, he or she will become the transcendent version of it, opening up the opportunity for stronger skills and larger stat bonuses. The maximum job level for transcendent job classes, unlike non-transcendent job classes, is 70.
picture from sitego.fr
Text source: StrategyWiki