Many kinds of trees set too much fruit. If you allow all of it to ripen, the fruits will be small and poor in quality. They'll also be more likely to suffer from pests such as codling moth, since closely set fruit provides a hiding place for undesirables. Certain diseases (apple scab, for example) may be more prevalent as well, due to decreased air circulation.
To encourage a healthier harvest of larger fruits (and to prevent overloaded branches from breaking), thin the crop when the developing fruits are about an inch in diameter. To avoid damaging branches, twist fruit off gently rather than pulling it. Thin apples to 6 to 8 inches apart, apricots to 2 to 3 inches, peaches and nectarines to 6 to 10 inches, and Japanese plums to 4 to 6 inches. Other kinds of plums, prunes, cherries, citrus, and pears usually do not need thinning.