They are harsh environments that require specific techs before you can colonize them. Each type of extreme world (e.g. Aquatic) has a level 1 and level 2 tech--you start out with a 50% production penalty until you finish the tech branch.
I have a casual impression that extreme worlds might be more likely to have a high PQ, but I'm not one of the spreadsheet builders around here. Loupindor, Sole Soul, or another of their ilk might have some firmer opions on that part of your question.
Also, some civs start out with an extreme environment tech, e.g. Torians and Aquatic worlds or Yor and Barren ones.