Thumbs up, thumbs down, smiley faces, yellow triangles with an exclamation point on it.  

How do particular threads get these icons applied to them and what the hell do they mean?

There's a regular smiley and a smiley with a green bar over his mouth, a gray rectangle, thumbs up, thumbs down, yellow triangles with an exclamation point on it.

Plus, who is applying these icons?