P/F or ranked has to do with what the grading schemes that the different schools use.
Ranked means that you are given a percentage grade/letter grade, so you are aware of your standing within the class. Some have argued that this is a benefit for applying to specialty programs, as you can clearly show that you are at the top of your class, if that is the case. However, people also say that this leads to more competition.
Pass/Fail (P/F) is just what it sounds like - you either pass or fail. A benefit of this is that it supposedly it decreases competition, because people are not trying to beat everyone else out (they just need to pass). Some schools apparently secretely rank you (in the event that you are trying to apply to specialty programs) however you are not aware of your ranking.
At some schools I have read that they have Honours/Pass/Fail, which is kind of in-between the two systems - it's like the Pass/Fail, but there is also a higher level of achievement in the "Honours" category.
Hope that helps! P.S. Anyone feel free to add anything/correct me if I am mistaken.