Acne Studios is the mainline brand, basically the brand known as Acne. Afaik "pop classics" was a lower priced alternative that they only did 2010-2011, pretty sure it doesn't exist anymore. I've only had one jacket from pop classics which was AW10 or AW11 (not sure if they actually ever did an AW11), and the quality was definitely worse than Acne Studios and kinda weird design, so I sold it again. Acne Jeans is just all their jeans offerings I believe.
On the topic of Acne I'm interested in what everyone thinks of the brand? Living in Denmark I see a lot to the brand - especially their jeans and scarves seem to be popular amongst girls. I have a college sweatshirt myself, which is really soft and nice to wear, and I also bought a piece of knitwear this season which seems decent enough.