Bloodthirst is a keyword ability.
Bloodthirst is an old mechanic, but it hasn't shown up until 2012 Core Set. Bloodthirst is an ability with different scales. So bloodthirst basically means, "Bloodthirst X: If an opponent was dealt damage this turn, this creature comes into play with X +1/+1 counters."
So if I have a creature with bloodthirst 3, then that means that when it comes into play, if my opponent was dealt damage whether by creatures or spells, it gets the counters. The bloodthirst can be triggered by Lightning Bolt, Prodigal Pyromancer, or any creature that attacked and hit for some damage. Now, life loss from cards like Hideous End doesn't trigger bloodthirst, because it's not damage. A card like Golbin Artillery will trigger bloodthirst.
In simple form, a spell card that you use that does damage to your opponent will trigger Bloodthirst. However, if a creature you control does damage to your opponent, Bloodthirst will again activate.
When to use it
- Creature on the battlefield*- if you want to play a bloodthirst card and you have a creature what you need to do is find a way to deal damage to your opponent, such as attacking. but be careful you don't wont to have to waste mana pumping or saving your attacking creature.(because on your second main phase (after you attack) you'll want to play your bloodthirst card).
- Spell that does damage*- If you have a spell that does damage you'll want to insure that you will have enough mana to cast both of them in one turn. however it does not matter when you cast them both as long as it in a main phase. Also to mention if it has haste as an ability as well, if your looking to attack play it during your First main phase so you can attack that turn.