The common fruit trees of Europe - apples, pears, cherries, plums and so on - don't have boy and girl trees.
Each flower has male and female parts: for fruit to be made, the female parts must receive pollen from the male parts of another flower.
In most cases, this means that a fruit tree needs a pollination partner.
This is another tree of the same general type (i.e. apple trees pollinate other apple trees) but of a different variety (i.e. a Cox apple tree won't pollinate other Coxs).
There is one important detail: the two trees must be in flower at about the same time.
There are 3 types of fruit tree when it comes to pollination:
These detailed lists of pollination partners are divided into groups to make choosing a partner easy:
Plum, Gage, Damson, Mirabelle, Bullace Trees - these will all pollinate each other.