Is it possible on adword to target one or couple specific site? Ex. If I were to advertise on DP forum, can I just target DP forums? so that it can only appear in that site?
Yes - it's called a Placement - but the site you target has to be in the adsense network (obviously), have quite high traffic, (can't remember the numbers, but if the traffic is below a certain amount it's not eligible), and not have opted out of being chosen in this way. And, the site can refuse to have your ad if it doesn't like the look of it.