A squire is a job who can not equip heavy armor, but can equip a sword, ax, knife, and flail. They are the starting block for all other jobs, they are average having no distinct weakness, but you probably won't have much luck playing with them throughout the entire game. Their ability is basic skill which is just a variety of abilities good in the beginning of the game but start to lack usefulness later in the game, but the main character, Ramza has some special abilities as a squire, his scream ability is very useful later in the game. Ramza's squire ability is labeled as guts and not basic skill.
A chemist is a job you can have very early in the game and who I suggest keeping as a chemist. Chemists are very useful all the way through the game. The can't equip heavy armor, and for weapons, they can equip knives and guns. If you learn all of the item abilities your chemist will be a very important part of your party. When a chemist gets a gun it, turns it from a harmless healer into a killing machine, with the longest attack range in the game (except for worker #8). A chemist will also make killing undead monsters a breeze
Knight is a simple job which can equip heavy armor and the oh so powerful knight swords. They usually have the highest HP in your party. Though heavy armor can greatly improve your defense, lighter armor sometimes improves strength when knights, believe it or not, some times lack strength. I like the break techniques knights have. This isn't good just for knights but for many other classes too.
An archer is the only job that can use a bow. They can equip light armor which has it's ups and downs. Archers are very good in the beginning of the game but as you get further into the game, they begin to madly suck. Their charge ability is good to build up and hold onto for later in the game, with a properly used charged attack you can cause the largest attacks in the game. The charge ability is better with other characters then the archer
A wizard can equip light armor and rods. Picking an elemental rod will strengthen the wizards magic for that element. The wizard is a great job, wizards can cause some huge damage with magic attacks, even very early in the game! There are some certain things you must do with your wizard to get the most out of their abilities.
A priest can equip certain rods and light armor. I wouldn't suggest getting a priest. They aren't very helpful to your to your party. The only way they could be useful is if they had all the protect, and shell spells, and the holy spell. Even with all those they're still sucky. The only reason to have them is to get an Oracle.
Monks have very strong attacks, some with a very wide range. They also can bring back dead units, heal all abnormal status with a single ability, restore a large chunk of HP and MP also in a single ability, and cause death sentence. They also have some of the best reaction abilities in the whole game ! Monks are all of these while being a very inexpensive job to have in your party. I suggest getting a monk early and allow them to grow into their full potential.
A thief is an all right job. They have a real large moving level, high speed, but low attacks. Their steel ability has its ups and downs. Steel heart and the abilities that you let you steel stuff equipped on your enemy are good, but I don't really think the rest are too good, or really that reliable. I wouldn't suggest keeping a thief in your party too long, the best thing that thieves do for you, is allow you to get other, better jobs.
Time mages are good jobs. They have a fairly high attack for being mages. They can equip staffs, and light armor. Their abilities haste, slow, stop, quick, don't move, don't act, and meteor, are very useful. The time mage ability is a good second abilities for members of your party. They also have the best move ability in the game - (teleport), which you might want to pick up for members of your party who don't have a high move level.
Oracles have the highest attacks of all the mages, and have an attack range of 2. They can equip long rods, and light armor. I don't like their ying-yang ability all that much. Paralyze is all right, the rest of the good ones take too long to get. I wouldn't have an oracle in your party for too long.
Geomancers are only good for one thing, getting other jobs. They can equip swords and light armor. They have all right attacks, and normal defense that you get with light armor. Their ability absolutely sucks. You have to spend a lot of time getting abilities that can work on a lot of ground, then when you do get them, the attacks only take away 10 to 60 hp at any point in the game!! Don't keep geomancers in your party any longer then you have to.
Lancers are a very good job. They can equip spears and heavy armor. The spears have an attack range of two, and are very powerful. Their jump ability is good, but sometimes it takes too long for them to come down. Lancers can become extremely deadly if the build up a long versatile jumping range. Lancers can be keepers
Summoners are good jobs. They can equip staffs and light armor. Their summon ability is great, it can cause very high attacks, status changes and recover HP. You will want to keep you summoner throughout the game, because the summon abilities cost a lot of AP. You will also want to make sure your Summoner has a very high level of MP, even at the cost of hp, because the summons eat up tons of MP, but the summon ability is worth it.
Mediators are not very good. Their best attribute is that they can equip guns. For armor they can only equip light. Their ability is terrible, they cause slight status changes that don't do much unless they are done many times. The other skills like invite, almost never work. I would only have a mediator in my party as long as I needed to to get a different job.
Samurai is a great job. They equip the katana as their sword, which is more powerful than a normal sword. Their attacks are more powerful then a knights attack, and their draw out ability can be powerful against multiple enemies, or beneficial to multiple allies. The Samurai is an expensive job, the draw out abilities will often break the sword that is being used so you might have to stock up on expensive swords just to use the samurai to their fullest, but I think a well used samurai is worth the cash.
Ninjas are the best job in the game. They equip flails or ninja swords which are comparable to normal swords in power. For armor they can only equip light which I think is good because light armor can improve your strength. Ninjas have the ability to use 2 swords in one attack, which doubles the damage. A strong ninja can take 450+ hp away in a single turn! Ninjas are also fast and have a large movement range which makes evading them difficult. I like their throw ability, it has good range and can cause some high damage. I suggest you get a ninja, and keep it!
Calculators are very good jobs. Their Math skill ability is extreamly useful. Even though they are a good job don't keep one in your party for very long because their Math skill ability is much more powerful when used a backup ability for a Wizzard. Math Skill is one of the most useful abilities in the game!