Time Mage is similar to your Black Mage and your White Mage in terms of prowess and stamina. They differ, of course, in the type of magic they wield. Time Magic can be extremely useful on a couple of levels: you can speed up those slow Knights of yours and slow down speedy enemies. They can give aid in terms of the Regen spell, and eventually, restore battles to very outset if they don't turn out to your liking. They can also cast powerful space time distortion magic which can seriously hurt anything on the recieving end.
Sorcerer is kind of like a combination between a Mage and a Knight. Technically they are mages, but they can equip most of the strong armor. Their key feature is that they can take advantage of an enemy's weakness by casting a certain spell onto their own sword. For example: remember those Ice Soldiers? Imagine combing the damage of your Knight's Mythril Sword with the damamge of Black Mage's Fire Spell. It sounds really good, but you have to waste a turn casting the spell on yourself every battle! This step is very time consuming, so I don't use them that much myself.
Berserkers are very powerful teammates, but you can never control their actions! They are under the constant haze of Berserk, which increases their speed and attack power, but turns them into raving nuts at the same time. Good for leveling up, but not all that useful in battles that count, or, for that matter, in battles that you're trying to derive something from (ie: getting Blue Magic, stealing).
The Red Mage is by far my favourite character at this point in the game because they have prowess over both White and Black Magic, and they can equip moderately strong attire and weapons. Later on in the game you'll want to switch jobs, but for now, all around is is the way to go.