All flowering plants require pollination in order to blossom and produce fruit. Some plants can pollinate themselves, in a process known as self-pollination, while most plants require pollen from another plant of the same species. This cross-pollination can sometimes occur between plants which are closely related, but are technically different.
What about pears and apples? Can pears pollinate apples?
No, they cannot. Apples and pears are not closely related enough to be able to cross-pollinate one another. A few varieties of apple are self-pollinating, but most will require pollination from another apple tree planted somewhere nearby. The specific variety of the other tree is also important, because not all types of apples are capable of pollinating one another.