You pay each day when you enter the park, or purchase a National Park Pass
http://www.pc.gc.ca/pn-np/ab/banff/visit/tarifs-fees_E.asp?park=1

No fees on trails.  You used to be able to hike up near the gondola, have lunch up there and then ride down for free - I assume you still can. 

It's just like every other national park I know of. You pay at the entrance for length of your stay. Be sure to have your camera out and ready during the drive because I remember seeing mountain goats and other animals all along the drive.

At the entrance means when you get to the park there is a "guard" station with a park ranger and you pay the man. It's not really that complicated.

If you go to Lake Louise, be sure to do the hike up to the "Tea House" (Do lots of leg stretches first or you will get sore legs - Don't do it if you have lung/breathing issues, as the air gets pretty thin - this coming from an Ontarian, who is way closer to sea level). The view is stunning though!

Also, try hit Morain Lake. It is almost prettier than Lake Louise.

Columbia Icefields are cool too, especially if its sunny cause you can get some good "under the edge of the ice" shots at the base sometimes.

In Banff, the elk come right into the town so keep your eyes peeled...

http://www.sxc.hu/photo/307972

Have Fun!!!

Thanks for the tips -- I'm heading to Banff later this summer and will use the info.