What is Bloodthirst?Edit
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.