The stack
Core concepts
Where spells and abilities wait to resolve — last in, first out.
How it works
Most spells and abilities go on the stack when cast or triggered, then resolve one at a time. After something is put on the stack, players get priority and may respond by adding more on top.
The stack resolves last-in, first-out: the most recently added spell or ability resolves first. Nothing resolves until all players pass priority in succession.
Common confusions
“I respond to a creature's enters-the-battlefield trigger by killing it — does the trigger still happen?”
Yes. The creature already entered, and the ETB trigger is already on the stack independently of the creature. Killing the creature in response doesn't remove the trigger.
“Do mana abilities use the stack?”
No. Mana abilities (and special actions like playing a land) don't use the stack and can't be responded to.
“Two triggers at once — what order?”
The active player's triggers go on the stack first, then each other player's in turn order; so the last-placed (non-active player's) resolves first. For the messy cases, see the tough-rulings guide.