How to make foliage particle on trees?

Hello,
So what im talking about is the way Mafia 2 (and unreal tournament i suppose?) is rendering trees foliage where leaf texture always looking at the camera. I guess its particles. Am i right? But Is it made inside the engine or do you set up something inside 3d modelling program? I just dont really get the way it works and i want to make the same thing for my game.

I was trying to find some tutorials or hints how to make it but all i could find is how to make falling leaves and thats not what i need.