How to create billboards in blender?

Let me start off by saying im talking about a physical billboard with advertisement but planes that are always facing the player.
I have been playing american truck sim for a while and noticed that the way they made things like trees was really smart and also took a lot let geometry then what I was doing before, instead of make a large clump of planes you simply use a billboard to represent those clumps, the billboard is locked so it can only rotate on a certain axis but it looks decent.
If anyone know how to do this your information would be greatly appreciated, I am only asking how to create billboards in blender to summarize.

 

Thanks for the quick response!