Popular is where the super fans (and hooligans) are on opposite ends of the field, behind the goals. One side is the home team and the other side is the opposing team. It's where all the songs and chants come from, and it can be fun or chaotic, but always really intense. It's the center of all of the action. The photos you see from River Plate stadium with the signs, fireworks, whatever, are all from the Popular. The tickets are the cheapest, the area is fenced off and heavily policed, and if there's violence, it's usually there. I seriously wouldn't buy tickets there unless I was going with a group, preferably of big guys. Platea is the area spread across the midfield, more expensive, with actually seats, better views, etc.