Pleiotropy is the condition in which a gene influences the phenotype of more than one part of the body.
A trivial instance would be that the gene influencing the length of the left leg also influences the length of the right leg. The growth of legs probably takes place through a growth mechanism controlling both legs.
Pleiotropy exists because there is not a one-to-one relationship between the parts of an organism that a gene influences and the parts of an organism that we recognize as characters.
Genes divide up the body in a different way from the human observer. Genes influence the developmental process, and a change in development will often change more than one part of the phenotype. This sometimes places a developmental constraint on the adaptation of organisms.
This Hawaiian happy faced spider illustrates the concept of pleiotropy. Here the length of matching legs is controlled by the same genes, giving the spider its symmetrical appearance.