There are two separate parts to Antelope Canyon: Upper, and Lower. Both are Navajo Tribal Parks, but the tours of each section are separate, run by entirely different tour companies. There are important distinctions between the two. The hike through Lower Antelope Canyon is longer, but the slot canyon is a good bit narrower, a tight squeeze in some spots, and to enter it, you have to descend steep metal ladders bolted to the rock walls. It's not difficult, but it wouldn't be advisable for anyone for whom mobility is a problem, or for anyone who has a problem with heights. As a result, there are fewer visitors to Lower Antelope. Both sections of the canyon have the same extraordinary, swirling sandstone formations, but the lower canyon has a more intimate feel.
If you go:
All visitors (other than Navajo tribal members) must pay a fee of $8.00 for a hiking permit, which is valid at both sections of the canyon. The parking lot for the lower canyon is right by the entrance, located just outside the Page city limits, off Highway 98, near the Navajo Power Plant. Visitors are not allowed to enter the slot canyon unaccompanied, you have to join one of the tours, which start every twenty minutes throughout the day. The cost of the standard one-hour tour is $30 per person (which includes the $8.00 fee). There are also special tours for photographers.
Links:

Indian Rte. 222
Page, Arizona 86040 USA
Phone Numbers:
Lower Antelope Canyon Tours
+1 (928) 640-1761
+1 (928) 640-1761