Smoke Screen is a Defensive Demon Hunter skill unlocked at Level 8. Using this skill allows the Demon Hunter to vanish from sight and knowledge of monsters for a second, allowing for escape or even an attack without the monsters reacting in any way. While monsters will not attack an invisible Demon Hunter, characters are not immune to damage while invisible, so this will not save you if you are standing in a lake of fire or other AoE effect.
This skill was widely used and abused in the days after Diablo III's launch, to the point that Blizzard applies a hotfix to reduce the invisibility time from 2 to 1 second.
 Skill Rune Effects
The following is a very quick summary. See the Smoke Screen rune effects page for a more thorough description of rune effects, or any of the individual rune pages for numbers, screenshots, blue quotes, and much more.
|| Smoke Screen
|| Cost: 14 Discipline|
Vanish behind a wall of smoke, becoming momentarily invisible for 1 seconds.
|| Choking Gas
|| Leave behind a cloud of gas that deals 700% weapon damage as Physical to enemies in the area over 5 seconds.
 Skill Design
The Demon Hunter's class concept included ranged attacks, traps, and movement and stealth skills, and being able to vanish is clearly an excellent form of stealth.
The following Demon Hunter passives provide a benefit to Smoke Screen, runed or not:
|| Tactical Advantage
|| Whenever you use Vault, Smoke Screen, or backflip with Evasive Fire you gain 60% movement speed for 2 seconds.
|| Reduces the Discipline cost of all skills by 10%. Increases your Life, Armor and all of your resistances by 10%.|
Discipline is used to fuel many of your tactical and defensive skills.
Smoke Screen was first seen at the July 2011 Press Event, where it was a level 12 skill. Unfortunately, no information about the skill's function was available.
When the beta started in September 2011, Smoke Screen was changed to level 18. The skill system was once again overhauled for the Beta Patch 13, and Smoke Screen was classified as a Defensive skill and bumped up to level 22. For Beta Patch 14, it was dropped to level 8.
Smoke Screen can be seen in action on video here:
You can find pictures in the Diablo III screenshot and picture gallery: