During battle, you will see symbols appear and disappear from underneath your dragons' HP bars and the enemies' HP bars as well. Multiple buffs and debuffs can be applied to one dragon. In some cases, a buff or debuff can only be applied once (e.g. Critical DMG buff or debuff), while others can stack up to 3 times.
Berserk applies to bosses only. It stacks 1 Berserk every minute, and can stack up to 20 Berserk.
This page is a glossary of what each symbol means.
Accuracy
Attack
Attack Speed
Berserk
Critical Damage
Critical Evasion
Critical Rate
Damage-over-time effect
Defense
Evasion
Healing
Health Points
Immunity
Recovery Rate
Shield, Invincibility, or Reflection Effect
Silence
Sleep
Stun
Zombie