Status ailments in Final Fantasy V don't work in a way that is as straight-forward as they might seem at first glance. Because of that, we've got a full description of what each ailment actually means and does to help make it clear.
Not a status ailment per se, Near Fatal provokes different behavior nonetheless. Near Fatal characters will be protected from physical attacks by Knights and those with the Cover support ability. The Shell status will be set on those with the Magic Shell support ability.
Darkness cuts the hit rate of all blockable physical attacks made by the inflicted monster / character by 75%. It also prevents the character from using the !Aim action ability. Darkness lingers after battle.
Poison deals non-elemental damage equal to the affected target's maximum HP/16. Out of battle, every step taken results into damage equal to the character's maximum HP/64. Poison will inflict no damage on a character with Stop set. Poison lingers after battle.
Mini doubles the affected target twice its normal Evade %, but reduces Battle Power to 3 (also for Throw and Goblin Punch purposes) and Defense to 0. Mini also affects the sprite of the affected out of battle, which is hilarious. A combination of the Mini status and the Genji Shield gives a character complete physical evasion. Mini lingers after battle.
Toad nullifies Defense, Magic Defense and Evade %. Battle Power is set to 3 (also for Throw and Goblin Punch purposes). All secondary abilities are blacked out except for !Black. Only the Toad spell may be used from the skill menu. Monsters under the effects of Toad may also use Toad spell. Toad also affects the sprite of the affected out of battle. Toad lingers after battle.
Silence prevents the usage of !Black, !White, !Time, !Spellblade, !Summon, !Sing and !Dark Arts and the spell listed thereunder. All other magical attacks can still be executed, most notably Blue magic and enemy attacks of all kinds. Enemies with Silence set may still use Dark Arts attacks.
Confused characters will turn against their allies, defaulting to a physical attack aimed at another character. If they are capable of casting !White, !Black or !Red spells, they may also cast offensive spells on your allies or helpful spells on your enemies. A Confused target cannot avoid physical attacks, not even with the help of Elven Mantles, Main Gauches, or Aegis Shields. Confused creatures automatically come to their senses when they take physical damage, but not magical damage. The only two exceptions are !Mineuchi and the last two hits of !Rapid-Fire. Confused characters will not Cover for other allies. Confused monsters will randomly use any of the (maximum) four attacks listed in its Control menu with reverse targeting, aiding you and harming other monsters. Counterattacks and normal AI attacks are not executed.
Paralyzed creatures cannot act, nor will they Cover for other allies. The duration of Paralyze depends on the attack setting it. A Paralyzed target cannot avoid physical attacks, not even with the help of Elven Mantles, Main Gauches, or Aegis Shields. Paralyze wears off automatically.
Sleeping creatures cannot act. The duration of Sleep depends on the attack setting it. A Sleeping target cannot avoid attacks, not even with the help of Elven Mantles, Main Gauches, or Aegis Shields. Sleeping creatures automatically wake up when they take physical damage, but not magical damage. The only two exceptions are !Mineuchi and the last two hits of !Rapid-Fire. Sleeping characters will not Cover for other allies. Sleep wears off automatically.
Characters under the influence of Old will see their stats gradually deteriorate to 1; their level will remain the same. Monsters will have their level and agility gradually sink to 1 for all purposes; attacks that check for level will look at this deteriorating level. Old lasts the entire battle until cured, but is automatically removed at the end of battle. Level 2 Old ignores Old immunity.
Berserked characters or monsters will deal 50% as much damage with their normal physical attacks, but are forced to execute a basic physical attack every turn. Berserked targets are immune to the effects of Confuse and !Flirt and the Lilith Rod, nor can they be the victim of !Control. Berserked characters will not Cover for other allies. Monsters will not counter attacks when Berserked, but characters with Counter set may still. Kiss Of Blessing ignores Berserk immunity. Berserk wears off automatically.
Image sets up a duplicate target that causes the next physical attack that would otherwise hit, miss. When Image is successful in helping avoid a physical attack, it will be removed. !Image, the Kiss Of Blessing and the Blink spell set two Images, the Mirage Vest only gives a single Image. Image status wears off after battle.
Float status lets the target avoid ground-based attacks; many monsters have inherent Float to simulate flying enemies. Ground-based attacks include Titan (Gaia's Wrath), Earthquake, Quake (from the Gaia Bell and Gaia Hammer), Squirrel and Wild Boar. Rockslide, Quake Shot, Quake Burst and Quake Cannon are all Earth-elemental attacks that disregard the Float status; note that many enemies with inherent Float are also immune to Earth-elemental attacks.
Regen periodically regenerates HP based on your Level and Stamina; skills that raise level or stamina also boost the HP healed by Regen. Regen will not heal a character with Stop status. Dragon Armor ignores Regen immunity.
Causes the ATB bar from both characters and monsters to take twice as long to fill. Mutually exclusive with Haste; Slow is removed when Haste is set and vice versa. Slow doubles countdown timers caused by the Doom spell and !Condemn.
Causes the ATB bar from both characters and monsters to take half as long to fill. Mutually exclusive from Slow; Haste is removed when Slow is set and vice versa. Haste halves countdown timers caused by the Doom spell and !Condemn.
Stopped creatures cannot act. The duration of Stop depends on the attack setting it. A Stopped target cannot avoid physical attacks, not even with the help of Elven Mantles, Main Gauches, or Aegis Shields. Stopped characters will not Cover for other allies. Timers will not go on; ATB bar will be frozen, Countdown timers, status duration timers, Regen and Poison will not take effect. Note that Sap will continue.
Targets with Shell active take only half damage from magical attacks, and magical attacks that are not unblockable have their Hit Rate halved as well. Magical attacks that are almost entirely barrier-piercing are still halved in effect; magical attacks that disregard Magic Defense due to elemental edges are still halved in damage, magical attacks that deal a set amount of damage are unaffected. Shell automatically ends after battle.
Targets with Protect active take only half damage from physical attacks. Physical attacks that disregard Defense due to an elemental edge or any other physical attack that ignores Defense completely is still halved by Protect. Protect automatically ends after battle.
The Reflect status allows certain spells to bounce off the target onto the opposing party. These spells will be reflected:
Sap causes the current HP of the target to drop by a marginal, absolute amount every interval. Stop will not stop the erosion. The duration of Sap depends on the attack setting it. Sap automatically ends at the end of the battle.
Characters with Zombie set are considered 'dead', and four Zombies gets you a Game Over. Zombies have their HP set to 0, and attack automatically, with basic !Attacks, directed at the party. Zombies are immune to Darkness, Poison, Float, Mini, Toad, Petrify and Death, and damage dealt to them have no effect. Zombies will not Cover for other allies. Zombie lasts after the battle, and can only be cured by Holy Water; Tents and Cottages will not heal this most unholy of status ailments.
Characters with Petrify set are considered 'dead', and a party of four statues equals a Game Over. Petrified characters keep their HP score, but can no longer act, nor can they Cover for other allies. You cannot target Petrified characters with conventional attacks; only those that are specifically designed to be able to hit characters with Petrify can be directed at them. Monsters with Petrify set are just killed, and cannot be healed. Petrify lingers after battle.
Characters with Death set are considered 'dead', and four characters with Death set gets you a Game Over. Characters with Death set cannot act, cannot Cover for other allies and cannot be hit with conventional attacks. Their HP drops to 0 if not already there. Though Death immunity is possible, Death immunity is disregard when HP is brought to 0, when the Death immune target uses an attack that kills the caster, when the Death-setting attack is one that destroys the target (as it checks for Heavy nature rather than Death status immunity). While revival attacks will normally 'heal' Death, they will set Death on Undead targets. Death lingers after battle.
Death is all.