Jobs

Jobs or classes define the profession of a person, allowing somebody to learn new skills at a faster rate. In theory, one could go against their assigned job, but in those cases it's easier to just change your job.

Leveling

A job level represents the amount of experience one has in their job. In most cases monster subjugation suffices, though each job has requirements based on the kind of job. For example, merchants gain more experience when purchasing and selling goods, and artisans level up based on their experience with their crafts.

Leveling up a job is mostly just for show, though there skills that are tied to the job level. Even though these skills are often unique to the job, once acquired it doesn't require the owner to have that job active.

Gaining jobs

People can only get certain jobs, depending on their own stats and history. Often it's as simple as leveling up an available job, though some have unique conditions.

Multiple jobs

Usually, one can have only one job at a time, however, an extra job slot can be granted, which allows one to have multiple jobs. Each guild branch can grant you one extra job slot, though there are some ways to obtain even more slots.

There aren't exactly any real benefits to having multiple jobs, other than to making it easier to level up a job.

Changing jobs

Jobs can't be changed without a skill. The most common skill used is 【Job Change】. This allows one to not just change the job of oneself, but also that of another. You can't change to a job that one doesn't know, and when changing the job of another, permission has to be granted. Also, when changing another's job, unless you know exactly what job is available, it's impossible to do it. There are artifacts and magical tools that help finding out what jobs are available, but it just suffices to know to what job that person wants to change.