Final Fantasy XIV uses a system called the Armoury System which allows a player to instantly switch between one class to another.
The extent of the jobs are still unknown such as will there be a form of Dragoon or Corsair from Final Fantasy XI returning.
THE ARMOURYThe Armoury is a method whereby equipping a certain weapon or tool will determine what you are.
For example, someone is currently a form of warrior using a sword (#01) and is attacking monsters outside of their home but then gets invited to join a party where their current skill with the sword is not good enough.
To solve this issue they put their sword away and equip a staff (#02) seeing as they are a seasoned mage that would be more suited to the mission. This change of weapon changes their entire combat setup from high attack physical damage to high magical damage.
Once the mission is over they wish to do some fishing so they unequip their staff and replace it with a fishingrod (#03) thus making them a Gatherer class.
After they caught all the fish they want they need to sort out some equipment and swords through their trusting blacksmithing skills so to do this they unequip their fishing rod and replace it with a hammer (#04) making them a form of Crafter.
This is the basic concept of the Armoury System or Job System for Final Fantasy XIV.
DISCIPLINESSince in Final Fantasy XIV your weapon determines your job, there are four main types of classes called Disciplines that you can either make balanced across all four or become a specialist in one form of Discipline.
Disciples of War are masters of arms and are the physical attackers in the world.
Disciples of Magic are the masters of spells and all things magical.
Disciples of the Land are gatheres of material resources and students of the natural world.
Disciples of the Hand are crafters who invest their creative ingenuity in synthesis and industry.