What are the uses of the various schedules of PVC pipe?
The schedule is the wall thickness of the pipe (essentially how much pressure it can withstand).  Wikipedia has a Nominal Pipe Size Table (and the article goes into the description of the size and the why behind them).

From this article http://www.ehow.com/facts_5575151_difference-between-schedule-schedule-pipe.html:


  For a relatively constant pipe outside
  diameter, the wall thickness varies
  from Schedule 10 (light) to Schedule
  40 (standard weight) to Schedule 80
  (extra strong) to Schedule 160 (double
  extra strong).
  
  Schedule 40 pipe is typically used in
  construction. However, if a stronger
  pipe is needed, then Schedule 80 can
  be used. An example where Schedule 80
  should be considered is if the pipe is
  exposed during construction. The
  stronger pipe is more resistant to
  incidental bumping by workers or
  equipment.


In my house the line that runs out to my irrigation system and the sewer pipe are all schedule 40 (the schedule is usually printed on the pipe).