The Active Denial System (ADS) is a giant heat ray (or pain ray as it's informally and less ambiguously known). The weapon fires electromagnetic radiation at the target causing a painful burning sensation.
It can penetrate through layers of clothing, though apparently can't pass through thick metal such as a trash can, so basically Oscar the Grouch should be fine but Big Bird is fucked.
Why It Won't Kill You:
The military says the ray only causes the impression of burning (they say your hair won't actually catch on fire, for instance). The ray just barely penetrates the skin, just enough to make your nerve endings think you're on fire. To prove this, they often invite people to stand in front of it.
Why You'll Wish it Had:
The military insists the weapon is safe because targets run away so quickly, and don't get a whole lot of exposure. Likewise, concerns that it can melt your fucking eyes were answered by pointing out how quickly people snap their eyes shut and turn away when blasted with the beam. So, what if you can't run away? Like, say, you are wounded because you're in a fucking war zone? We suppose you're going to lay there and feel like somebody stuck you in a microwave oven.
Not to mention that all of the safety testing was conducted by the army. The same people who want a green light given on the weapon's safety. So the testing procedure probably went like this:
The Army: Is that weapon safe?
The Army: Yes.
The Army: Excellent.
That problem is all of the scenarios that don't come up in testing (for instance when cops found out that tasers plus pepper spray equals fire). So the military will have a lot of explaining to do when the ADS crosses streams with a PHASR up there and sends the victim into another dimension.