The Magus is one of the endgame jobs of Final Fantasy III. It is behind the Earth Crystal, alongside the Summoner and the Devout. It can be considered the equivalant of the "Black Wizard" from the original Final Fantasy.
The Magus is the "advanced" version of the Black Mage job: It can use Tier-8 spells and their middle tiers have far more charges.
In the original Final Fantasy III, the Magus comes at a somewhat awkward time. The job is located in the Ancients' Maze, right before the Crystal Tower, and by extension, Eureka. Eureka gives out the Sage job, which more or less outclasses every other magic job in the game. You don't need to use it, but you'd be crippling yourself not to.
Due to the Sage being nerfed to be on more equal footing, the remake's rendition of the Magus gets more use. However, while it is a straightforward upgrade to the Black Mage, it still starts at Job Level 1, meaning that leveling the job level will be necessary to get the full benefit. Thus, how hard it outclasses the Black Mage is somewhat up for debate. Curiously, the remake's Magus has a bell curve-ish design in terms of its MP growth - meaning that tiers 5 and 6 have the most MP in most cases.
The Magus has access to all tiers of Black Magic. This includes Level 8: which features Flare, Meteor and Death, the former two being some of the strongest attacks in the game.
The elemental rods can allow the Magus to maintain the ability to use the tier-2 elemental spells. Quake can be useful for a time, but as most enemies in the late game resist earth, it will require relying on Bio or Meteor by that point. As with the Devout, the late game rods (Wizard, Omni and Millenium) are recommended.