Job System

A variation of the Class and Level System that's most often seen in Eastern RPGs, where instead of a character being locked into a single class for the entire game, the player is free to switch each character between distinct classes ("jobs") at their leisure, to adapt their party to different situations as the game progresses.

The ability to change a character's job is usually accessed through the party's menu outside of battle (though some games may instead require visiting a specific location to change jobs).

Additionally, each job has an associated Level independent of the character's own Experience Level, which dictates their proficiency at the assigned job and unlocks new job skills and/or abilities as it levels up. For example, if you've never put a character on Mage duty, the will have a minimum Mage level (and only beginner-rank spells to use), whereas a character who has reached maximum Mage level will have a wide arsenal of magic at their command.

Another common feature is the ability to mix-and-match a limited amount of skills from one job onto another, such as allowing a Warrior to wield White Magic and heal his comrades during battle, or allowing a Mage to equip a sword and shield instead of the usual staff or rod.

Exactly which jobs a character has access to varies: Sometimes the job system is completely freeform and a character can change to any available job, while other times it is heirarchial and more powerful jobs must be unlocked by meeting certain prerequisites (such as mastering a the lesser jobs first).

Skills are generally handled in one of three ways -

Skills are exclusive to the class - i.e. if a Warrior learns Guard at Level 5, only a character who is a Warrior at Level 5 or higher can use Guard.
The character can assign the skill to one of their skill slots once they level up the job - i.e. anyone who levels up their Warrior class to Level 5 can assign Guard to their Skill Slots.
The class must be maxed out before the skill can be assigned - i.e. a Level 5 Warrior can use Guard, but Warrior must be maxed out before the character can use it.

Finally, when a character levels up, their current job may have an effect on their statistics—this usually falls into three categories:

Stats are tied to the class: A level 20 character in the Warrior class will always have the same stats as any other, and the Job Level is used to provide other effects.
Stat gains are tied to the class, but the actual stats are tied to the character. If a Warrior gains +5 HP, +2 MP, +1 Attack per level-up while the Mage gains +2 HP, +5 MP, and +1 Intelligence per level-up, a character who has been a Warrior for 20 levels before changing to a Mage will be less squishy than another who has been a Mage for the same 20 levels; however, their lower MP and Intelligence will hinder their effectiveness as a Mage compared to the other.
A character has a set of 'base stats' independent of their job, but the job provides additional bonuses on top of these; such as a Warrior getting a 1.5x boost in attack power while a Mage gets a 1.5x boost to their magic power.

Compare Stance System.

Examples of Job System include:

Anime and Manga

This is how things work in the RPG Mechanics Verse of Kono Subarashii Sekai ni Shukufuko o! The jobs that are available for a character are determined by their stats. For example, Kazuma has mediocre stats in everything except Luck and so can only become an Adventurer.

MMORPGs

The MMORPG Dream of Mirror Online uses Type 1, so different jobs have different stats. But you can use skillsets from other jobs, with some mechanical limitation: you can't use skills that are ten level higher than you current job level, and efficiency is reduced when using skillsets from very different jobs (like spell casting if you are a sword fighter).

Role-Playing Game

Final Fantasy is the Trope Namer here. Games from the main series featuring Job Systems include Final Fantasy III, Final Fantasy V, Final Fantasy X-2 and Final Fantasy XIII. It can also be found in all three of the Final Fantasy Tactics games.
- Statistics-wise: X-2 used the first method; III on the NES used the 2nd system but in the DS remake, it switched to the 1st system for everything except HP; Tactics used the second, and V used the third with the addition of Freelancer's having the highest stat boost for each stat in any of the classes they've mastered. XIII falls under none of the above; characters do not level up, but gain Crystarium Points to be distributed manually.
- Skill-wise: X-2, XIII, and both versions of 'III used the first scheme; Tactics used the second with the limit that you could only use your current job's skillset in addition to the ones mastered as one other class in addition to a passive ability, a counter ability, and (in the original Tactics) a movement ability; V used the first method, except when you played as Freelancer you had all passive abilities (except Berserk) applied and your choice of any two active skills.
- Final Fantasy XI uses a version of type one, which is interesting seeing as it's an MMORPG and most MMOs tell you flat-out "An Adventurer Is You" and give you no recourse to change your class after you create your character. There are around 20 different jobs, though you only start with 6, and you can change them at any time by going to your Mog House. You can even pick a secondary job to complement your primary one after a certain point in the game.
Some Dragon Quest games have it too: Dragon Quest III established the basic system, then it was expanded upon for Dragon Quest VI, Dragon Quest VII and Dragon Quest IX.
Blue Dragon is basically Final Fantasy V's job system.
The Tales of the World spin offs of the Tales (series) has them too, an interesting variation, since Tales of the World is an action RPG. Also, several "classes" are main characters from other Tales games.
The Ogre Battle series, of which Tactics Ogre, Final Fantasy Tactics' Spiritual Predecessor, is a part of, has this.
Golden Sun's class system, which relies on Mons and the odd Upgrade Artifact.
Wizardry reset your level and stats when you changed class, but not your HP or your spell list, which was all that really mattered for many classes.
Monster Girl Quest Paradox Job system uses the first method for skills and the third method for stats. The game also has passive abilities (e.g. a 50% resistance to fire damage), which can also be learned from jobs. Once an ability is learned it can be used regardless of the job.

Tabletop Games

Oddly enough, Dungeons & Dragons Third Edition is pretty damn close to being a Job System by this definition. Though the EXP penalties (that most groups don't bother with) are there to keep too much abuse.
- Pathfinder, with its usual preference for carrot over stick, eliminated experience penalties altogether. As with earlier editions of D&D, it's usually better to master one class than spread yourself too thin.

Turn-Based Strategy

Wild ARMs XF
The Reclass system in Fire Emblem: Shadow Dragon allowed for units (except Marth, ballisticians, thieves, and manaketes) to change into other classes, though there are some restrictions like the class selection being limited to three sets of classes based on gender and the unit's initial class, as well as limits as to how many units can be that specific class (How many units that join as that class initially + 1). When (ab)used with certain characters such as Wolf or Sedgar, this has the potential to make the the unit a walking Game Breaker by inheriting the class's base stats and gains into the units own, eventually leading to certain units with insane stats come endgame.
Vandal Hearts has each character start as one of four basic classes, Soldier, Archer, Healer and Mage. When they reach level 10 they can choose to become an upgraded form of their current class or switch to a more specialised type. Soldiers can become "Armours", with huge attack and defense but terrible movement and magic defense that makes them useless. Archers can become "Flyers", who move fast, ignore terrain and have high attack at expense of a massive weakness to arrows, and are fairly useful if a bit fragile. Healers and Mages can choose to become a "Monk", with average stats everywhere, the inability to equip good defensive gear and a hodgepodge of middling healing and supportive spells and low level attack spells with a magic power that never really raises above the base class'. The Hero also has his own unique class, the Hero -> Champion -> Paragon. But obtaining the Seven Holy Prisms (requires at least one flyer) and then completing the Seven Trials of Torroah unlocks his super secret class the Vandalier. It uses unique equipment, has massive stats in every area, knows every spell your team can learn and blocks all attacks from the front and sides. You only get to use it for about three or four fights though.
Unlike its predecessors, Valkyria Chronicles III allows every unit to change class, although each unit has one or two classes that they excel at.