Draw a line from the center of the attacker's space or burst to the center of the target's space.
Lesser Cover +1 circumstance bonus to AC if line passes through creatures but no objects.
Cover +2 circumstance bonus to AC, Reflex saves against area effects, and Stealth checks to Hide or Sneak. You can use Take Cover to increase this to greater cover.
Greater Cover As cover, but a +4 bonus.