There are buffs (positive) and debuffs (negative status effects). Here is a list of the different status effects, along with detailed descriptions of their purposes. 

Green status effects cannot be removed by other effects.

Positive Status Effects (Buffs)

IconIdentifierEffect
ATK UPIncreases attack by 25%.
DEF UPIncreases defense by 45%.
CRIT Chance UPIncreases critical chance by 20%.
CRIT Damage UPIncreases critical damage by 50%.
ACC UPIncreases accuracy by 30%.
RES UPIncreases resistance by 35%.
AGI UPIncreases agility by 30%.
CRIT ResistReduces chance to be critically hit by 25% (reduced from attackers critical chance).
InvincibilityCannot be damaged.
ImmunityCannot be debuffed. No negative effects can be applied.
RecoveryRecovers 15% HP each turn.
PreservationKeeps continuously at least 1 HP when damaged.
ShroudCannot be selected as primary target. Will only be hit by AOE.
Guard25% of the received damage is shared together with the caster of this effect.
ShieldReduces damage based on shield strength (determined during casting).
AbsorbingHeals 15% of the damage dealt.
BlessedIncreases amount of received healing by 20%.

Negative Status Effects (Debuffs)

IconIdentifierEffect
ATK DownDecreases attacks by 25%.
DEF DownDecreases defense by 45%.
AGI DownDecreases agility by 30%.
CritC DownDecreases critical chance by 20%.
ACC DownDecreases accuracy by 40%.
Res DownDecreases resistance by 15%.
Blind30% chance to MISS.
MarkedIncreases received damage by 25%.
SickCannot be healed.
CursedCannot be buffed.
StunnedCannot take action at the next turn.
SleepAction points loads 50% slower. This results in the hero's turn coming up less often.
FreezeCooldowns of skills are not reduced.
PetrifiedThe next hit received is a guaranteed critical one. Effect is removed after it hit.
BurnRecevies 3% of the targets maximum HP damage when attacked.
PoisonReceives 10% of the targets maximum HP as damage at the start of the turn.
BleedingReceive 10% of the caster's attack as damage at the start of turn.
MuteCan only use the basic attack (first skill).
BoundDeactivates passives.
TauntCan only attack the caster with the basic attack (first skill).
ConfusedIgnores the primary target.
Cursed FlameReceived healing is converted to 50 damage instead.

Hero specified status effects

IconIdentifierEffect
HellFire

Prellbog's unique skillset. Enables Hellfire Mode for 2 turns.

Hellfire ModePrellbog's unique skillset and increases +20% attack and +40% agility.
Frozen Heart

Each stack of Frozen Heart increases the Undead Dragon's HP by 15% (on top of the regular 50% max HP) when reviving. Limit: 5 stacks.

Heating Up

Construct's unique skillset. All enemies damage 60% ATK + 20% for each stack of Heating Up. Inflicts Burn if there are more than 3 stacks of Heating Up.

Mjöllnir

Thor's unique skillset. Single target damage 120% ATK. Inflicts Mjöllnir. When Thor has it’s turn again, Mjöllnir returns and inflicts 80% damage on targets with Mjöllnir.

Seal Anubis' Seal

Anubis' unique skillset. When an enemy with Anubis' Seal dies the entire enemy team receives 50% damage of Anubis' ATK.

Soulmate

Aphrodite' unique skillset. All damage Aphrodite deals heals her Soulmate by 5% of the damage dealt.

Sphinx Curse

Used by Sphinx to mark targets for a dangerous attack. Limit: 1 stack.

Styx

Reduces the target's max HP by 8%. Only removed if Hades dies. Limit: 3 stacks.

Burrow

Increases certain skill effects of the Ice Worm. Limit: 1 stack.

Defense Drone

Defense Drone increase Murder Machine's defense by 3% per drone. Limit: 10 stacks.

Divine Shield

Increases damage resistance per stack.

Ferryman

Increases the attack power of Hades' passive secondary skill "Cerberus".

Feral

Increases damage and AGI for 5% per Stack. Limit: 6 stacks.

Wildfire

Increases damage and critical damage by 5% per stack. Limit: 20 stacks.

Fury

Increases Bastet's critical chance by 5 per stack (limit: 5 stacks). If Bastet has at least one stack of Fury, she counterattacks with her primary skill when suffering damage, and one stack of Fury is removed.

Hivemind

Increases AGI by 5 per stack. Limit: 6 stacks.

Hot Saw

Adds a 80% Chance to inflict Burn when attacking with the primary skill.

Loa

Increases the RES by 10 per stack. Limit: 5 stacks.

The Moon

Inflicts Blind for 1 turn when attacking. Grants a Shield for 60% of its Max HP to the ally with the lowest HP.

The Star

Inflicts Mute for 1 turn when attacking. Boosts a random ally's AP by 100% when discarded.

The Sun

Inflicts Marked for 1 turn when attacking. Heals the weakest ally for 50% of its max HP when discarded.

Wind Up

Increases damage by 5% and Crit Damage by 5%. Limit: 6 stacks.

War Forge

Increases ATK by 10% and DEF by 30%. Stacks with ATK UP and DEF UP.

Drumbeat

A special Ability from the War Bard. Increases AGI by 4 per stacks. Limit: 4 stacks.

Foresight

A special Ability from Odin. Increases damage resistance by 15 per stacks. Limit: 4 stacks.