I was playing a game online the other day and a question about los came up. He said you can use all powers such as outwit, perplex, and pc, if the person you are using them on is through trees, or other hindering terrain. Is this true does hindering not block los for those powers which say you must have a clear los.

Hindering only blocks LOS if whoever you're trying to have LOS to has Stealth. Otherwise, figures without stealth/Batman Ally/Kabuki, can be seen through trees/bushes/other hindering normally.

The only thing that blocks Line of Sight is Blocking Terrain (i.e. thick black lines).

Stealth and the Batman Team Ability on the other hand state that any Line of Sight/Line of Fire that crosses Hindering Terrain is considered to be crossing Blocking Terrain. So Outwit, Perplex and PC can't be used on a character while they are in Stealth, even if the character using those powers is adjacent to the Stealthed character. This is because even though you are adjacent to Mr. Stealth, those powers say you MUST draw a LOS/LOF to the character.

Woo... that took longer than I expected

just to back-up what has already been said:

As it says, clear LOF is any LOF that is not blocked. The only things that block LOF are characters and Blocking Terrain. Hindering Terrain, by itself, does not block LOF.