A Turn is when a hero or group of monsters acts. A Round is when all creatures have taken a turn. Creatures act simultaneously “in-world,” so Rounds and Turns both take roughly 6 seconds. An Encounter is all of the rounds in a particular combat.
1/Turn Abilities. If you perform one on your turn, and can find a way to perform it on another creature’s turn, you can do it again (e.g., Sneak Attack combined with Opportunity Attack or Coordinated Strike!).
1/Round Abilities. These reset whenever your own turn ends (e.g., when you Defend, you cannot do so again until your next turn ends).