Fine I'll explain the classes for you.

Geomancer - The first time the class ever appeared and it's a doozy. Terrain is a weird ability in this entry. There are eight abilities in total based on the eight terrains. Three do damage to all enemies, two do damage to one enemy, the other three will instant kill one of them. The major drawback of this class is that if the Terrain ability misses all enemies, the Geomancer will be hit with Backfire, which does damage based on 25% of the Geomancers max health. When the moves land, the class is a beast, but the backfire effect is a real detriment, especially when the instant death stuff has the highest failure rate. The class is best used Underwater, in Caves, or on the open field because these three terrains do the group hitting attack and have the lowest backfire potential. They are also the only magic class with the Defend command.

Dragoon - First appearance in the series and not quite the beast this class is in later installments. Dragoons have their jump ability as usual. Jump can actually miss, which may seem odd for some people. Jump does higher damage but not necessarily the 2x people are used to and spears don't garner any more benefit for it like later installments. There are also fewer Lances in the NES version than the DS one. It's best attribute is that Jump still allows a character to dodge attacks which is the main reason why it's so damn useful for the Garuda fight.

Viking - Another tanky class like Knight. They come with the Defend command and are decent fighters. the original incarnation has higher vitality than their DS counterpart, but lack the DS versions higher strength. They are not bad units, but Knight largely covers what they do. You may want to use Vikings during the time where Knight gear is scarce and then switch back when the endgame knightswords arrive.

Magic Knight - Doesn't really shine until the endgame. They come with the ability to use low level white magic barring Aero. There main claim to fame is being able to use Samurai Swords which are the only physical weapons that can injure dividing enemies without making them divide. You'll want one or two of them for the Cave of Shadows and likely carry one into the final areas until Ninja is unlocked. They are an okay class in the defense department and thankfully the "dark blades" are generally stronger than a lot of Knight swords. The issue is that you need the Invincible to get this class any gear in the original. The DS gave them access to a few Knight weapons.

Evokers - This class is way better in the NES version. Evokers can soft summon the game's main summons. Basically they will either cast a Light or Dark Attack. Dark usually does some kind of elemental damage while Light is usually a debuff though some have other effects like healing, buffing, and physical damage. The elemental attacks only hit one enemy but do decent damage on par with a Black Mage. Where the NES shines over the DS version is that the status spells have 100% accuracy in the original, making this an ideal class for crowd control. Shiva's Paralyze skill or Leviathan's Petrify ability will do wonders for you in the Cave of Shadows.

Bard - The first incarnation of this class isn't so bad. In fact with the exception of Str. and Int. the DS version actually lowered the stats of this class for some reason. They are still on the flimsy side thought and Harps are not terribly great weapons. Their two commands are Scare and Cheer. Scare will lower all enemies level by 3. If use it enough time, enemies will actually run away. Bosses are of course immune to it. Cheer raises the whole party's physical attack power and will stack. In fact I don't think it even has a limit. So yeah, Bard's are neat.

Black Belt - Oof, easily the powerhouse of this group. They are an upgraded monk who get the Buildup skill. They are not quite as broken as their DS counterpart (which can one shot Cloud of Darkness and break the damage limit) but they are still one of the strongest classes in the game. There is a reason the DS version moved this class to the final crystal. The biggest issue with this class is their abysmal magic defense. Like seriously, their MND and INT will never go higher than 1 so they take lots of damage from magic. the class has the highest vitality in the game, so they are the perfect class to grind in for HP gains, especially if you've had a character stay a mage for too long.