Bees! All tree fruit needs to be pollinated by bees. If you plan to grow fruit, plan to deal with bees! While we do not recommend you go into the beekeeping business, it might be a interesting sideline endeavor, if you have difficulty finding hives to rent from commercial beekeepers, or your populations of wild pollinator bees is low. As you may have read, honeybees in particular are facing great challenges with colony collapse and other diseases and issues. Proper pollination and, hence, healthy bees are very inter-twined.
Pollination occurs when the blossom is open and the pollen is transferred from the anthers to the stigmas of the flowers. Since most fruit trees require cross pollination with a different variety, this pollen transfer does not always occur in the same flower. Often this transfer is not by a single bee visiting one variety and them going to another variety, like he "knows" that is his job. Often is by certain bees in the hive working a single tree over, flying back to the hive to "unload" their pollen and nectar, and rubbing against their buddies and transferring pollen in the hive. Then these bees take this pollen to another variety and to other trees. Bees are working to collect nectar and pollen for their own purposes, and the pollination a by-product that benefits the fruit grower.
Cultured honey bees are not the only pollinators in the world either! There are many native, wild bees of species very different from the common honey bee that are often much more efficient pollinators than honey bees. Learn to respect, maintain and enhance their habitats, since the commercial honey bee has many diseases and predator insects which have been affecting them in the past 15-20 years.
Pollinator Varieties: Many new and beginning fruit growers worry unduly about proper pollination---which varieties to plant next to others, how far apart they should be, when they bloom, etc. It really can be simplified more and made less frustrating.
As a general rule of thumb, if you have planted lots of variety diversity within a class of fruit in the same orchard, then you will often have adequate pollen sources, but if you have large blocks of varieties separated by 75-100 feet from each other, then plan to provide pollen sources within the blocks.