PPR or Passport Request is when the appropriate visa office requests that you mail them your passport so they can insert the 'permanent resident' visa stamp. (Note that as of 1 Dec 2011 this is changing a bit). This is the indication that they've processed your application for PR and have accepted it.
When they request, you send them your passport through the mail. They return it to you with a visa stamp and instructions on how to 'land' in Canada. That then allows you to go to the border and 'land'. At the landing (which I haven't yet done), it appears that the border officer asks you some specific questions to verify information on the application and to confirm that nothing has changed. I think this starts the process to get your PR card. Once landed, you are a permanent resident and can live, leave, reenter and work pretty much as a citizen can. (Anyone else, please elaborate on my simple understanding.)
As regards the visa stamp changes, there's a lot elsewhere on this thread about that. After 1 Dec they were going to do away with the need to send in the passport, and just have the border officer pull up information on their computer. They seem to be in transition. I was asked to send in passport after 1 Dec. I received a form called a COPR (Confirmation of Permanent Residence). I did not receive a visa stamp. I'm to take the COPR to the border and 'land'.