To produce fruit ALL fruit trees must have proper pollination from bees and other insects. 

Apples, pears, and sweet cherries must have other different varieties blooming closeby in order for the bees to transport pollen from one bloom to another allowing fruit to be produced.  Many apricots and plums also require cross pollination, although some varieties are self-fertile.  

In general though, peaches and nectarines, and tart cherries are self-fertile and a single variety can stand alone and pollinate itself.

For a more thorough explanation, check out "Pollination Basics" in the Growing Tips section.