Card sounds may differ from in-game as they get updated.
Deadly - Units and spells with Deadly kill any unit they damage, regardless of health. For example, a 1/1 with Deadly can kill a 5/5 attacker.
Killer - A unit with Killer can make one special attack directly against an enemy unit. This can be done the turn it's played, and can target any unit, regardless of Flying, Aegis, or other special abilities.
Revenge - When a card with Revenge dies, it permanently loses Revenge and gains Destiny and then it is put randomly into the top ten cards of its owner's deck.
Contract - You may pay the number in additional power to get its bonus. If you don’t have enough power to pay some or all of the Contract, the remaining balance is applied as “debt” that is paid at the start of the next turn.
Debt - Your available power is spent to pay off Debt. If you can't pay it all, you pay at the start of your next turn.