Creatures can have many different abilities, some of which can be gained from magic items, spells or the abilities of other creatures.  Not all are beneficial, such as being poisoned.  Creature abilities can take effect at various times during the duel.  These include:

On cast

The ability activates when the card is cast after the creature enters play.  If the card is disrupted, this ability will not activate.

Start of Turn

This ability will activate at the start of each of your turns.

Before Attack

This ability activates just prior to your creatures attacking.

End of Turn

This ability triggers at the end of your turn.

On Death

This ability activates when the creature dies.

On Equip

Activates when the creature is equipped with a magic item.

Receive Damage

The ability activates every time the creature loses life.

Damaged Creature

The ability activates when this creature damages an enemy creature.

Damaged Player

This ability activates when the creature damages the enemy player.

Killed Creature

This ability activates whenever this creature kills another creature.

Creature Died

This ability activates when any creature dies.

Creature Cast

This ability activates whenever a creature is cast onto the battlefield.

Aeon Abilities

These abilities activate whenever the player spends an Aeon card on the creature of the proper color.  The card is dragged onto the creature, instead of discarding it.

Continuous Abilities

Some abilities are always active so do not activate at a particular time, such as Armored or Venomous.