--Main Temples, Bangai Temples, Okunoin--
As you have already read, most likely, this pilgrimage consists of 88 temples spread around the circumference of the island of Shikoku. These aren't the only temples on the island, though; there are hundreds of other temples. Yet, all of the temples on the island fall under one of the following headings: Main Temple, Bangai Temple, and Okunoin.
The number 88, when referring to the number of temples, first appeared in 1471 on a waniguchi bell at a Jizo hut in Honkawa Mura, in what is now Kōchi Prefecture. No one today knows whether the 88 temples referred to at that time are the same as the 88 temples that constitute the pilgrimage today. In any case, 88 temples have been designated as the main pilgrimage temples and these are the only temples the vast majority of pilgrims visit.
Bangai Temple and Bekkaku Temple
Over the course of time, 20 of the bangai temples have become associated with the pilgrimage as well. Most literature and maps that i have seen, simply refer to these 20 temples as bangai temples, ignoring the fact that there are really hundreds of bangai temples around the island. As you walk around the island, other pilgrims will simply refer to them as Bangai 1, Bangai 2, etc.
The correct terminology for these 20 temples, though, is bekkaku temple. Bekkaku has the meaning of being associated with, related to, something else, while being secondary in some respects. For example, a hotel chain may have a main hotel in some city, and a bekkaku hotel nearby to house guests in peak seasons, when extra space is required. The service is about the same, the rooms are about the same, but it is not the main hotel, it is the bekkaku hotel.
Similarly, these 20 bekkaku temples are officially associated with the pilgrimage, although at a secondary level. They are considered pilgrimage temples and are found in most guidebooks, but it is understood that most people won't visit them. In fact, i don't think the majority of pilgrims ever visit them; most of the time i visit one i am the only person around. Nevertheless, even if they are off the main circuit physically and mentally, you can still get temple stamps (in special stamp books) at each of these temples, and you can purchase a small bead at each stamp office which can then be made into a wrist mala through an office in Kyōtō (i think).
Okunoin (Inner Sanctuaries)
The inner sanctuary houses a deity and has a legend attached to it that are related to the establishment of its associated main temple, so is considered the heart of the main temple. The okunoin is not located in the main temple compound but is usually either a small structure on the top of a nearby mountain or is another separate temple altogether. In some cases, one of the bekkaku temples serves as the okunoin of one of the main 88 temples.
Truth be told, almost no one reading these pages will ever visit an okunoin; even Japanese pilgrims seldom visit them. For those hoping to do so, special tours are arranged. For the vast majority of pilgrims a walk of the henro trail still means walking only to the main 88 temples.