The website for Canadian Nat'l Park reservations is Campground Reservation Service
The system is fairly simple. Right now, it doesn't show campground opening and reservation dates for 2011. That info will likely be posted after the first of the year.
You select a campground in the rockies parks, but not specific sites. You just enter your desired dates, equipment type, campsite type (hookup, no hookup, etc.). Whistlers, Wapiti and Wabasso take reservations in Jasper. The Lake Louise and Banff CG's also take reservations (don't remember specifically which ones, the website has that info).
My experience is that the full hook up sites in Jasper are available for at least 3 -4 hours after the reservation window opens in the spring. Avoid the hook up sites in the Wapiti CG - they are a gravel parking lot.
Other CG's along the Icefields Parkway are first come, first served. No hookups. Firepits are available, but there is a "fire permit" fee, $8.80 per night, which includes firewood.