It stands for Personal Ticket Office, AC's proprietary booking system. You can access it through http://www.aircanada.ca/pto/home.html

When you go there it will tell you that you must have been contacted by AC in order to use it. Ignore that and register then download. I didn't have any problem as an Elite. You will need to contact PTO to have your account activated, but that is straightforward. It's not perfect, but I prefer it to ITN except where I need to see what booking classes are available.