The Knight job belongs to the second tier of jobs, unlocked upon encountering the Fire Crystal.
The Knight job was also present in the original Final Fantasy. There, it acted as an upgrade to the Warrior job; In additional to an expansion to the weapons and armor that are available, the Knight had the ability to cover allies that were low on health. That's about the same as its portrayal in Final Fantasy III
The 3D remake makes a few changes to the Knight. In particular is its relation to the Warrior: In the original, the Knight was an upgrade over the Warrior. In the remake, now that the Warrior was buffed to be a glass cannon attacker that can work until the end of the game, the Knight was repositioned as a defensive wall. In addition to its original defensive properties, the Dark Knight's White Magic was moved to here on top.
Defend is the Knight's unique variant of the Guard command. Compared to Guard, Defend uses the job level while factoring in its effectiveness, starting at 40% and escalating by one point every 5 job levels, capping out at 59%. The more immediate benefit, however, is that the Defend factors in physical and Magical attacks, rather than just physical, making it useful against magic-wielding enemies
When an ally reaches 25% HP, the Knight will instantly block physical attacks towards that character, taking the damage in their stead. However, this technique does not activate for multi-targetted attacks nor magic attacks.
The Knight gets a lot of mention in the strategy guide due to its defensive capabilities, especially when it comes to their cover ability. Because of the protection this can give, the guide recommends fielding a Knight and a White Mage to aid in experimenting with riskier jobs. A more subtly clever way of using the Knight is to take advantage of the Knight's slowness to ensure that they move last, and therefore lowering the likelihood of a party member you're trying to raise getting dropped before they can be healed, among other things. However, Knights should not be fielded with Red Mages, due to the Red Mage's recommended loadouts requiring swords that the Knight would get more benefit from.