Area of Effect: A close attack creates an area of effect, usually a blast or a burst. A close attack affects certain targets within its area of effect, which has a certain size. A close attack’s area of effect and targets are specified in its power description.
Origin Square: A close attack’s area of effect defines the attack’s origin square, which is the attack’s starting point. A close burst uses your space as its origin square. A close blast uses a square within your space as its origin square. For a target to be affected by a close attack, there must be line of effect from the origin square to the target.
Multiple Attack Rolls but One Damage Roll: When you make a close attack, you make a separate attack roll against each target in the area of effect, but you make a single damage roll that affects all the targets. A Large or larger creature hit by a close attack is affected only once by the attack, even if multiple squares of the creature’s space are in the area of effect.
If you’re using a projectile weapon to make a close attack, you need one piece of ammunition for each target, and if you’re using thrown weapons, you need one for each target.
Published in Player's Handbook, page(s) 271.