In X-linked disease, the genetic defect is located on the "X" chromosome and usually affects males only.
This happens because females have 2 X chromosomes - 1 each from the mother and father, whereas males only have one X chromosome, inherited from their mother (they get a "Y" chromosome from their father).
Females with one normal X chromosome and one mutated X chromosome generally do not manifest the disorder because of the presence of the normal gene. However, these females are at risk for passing on genetic disease and are thus called "carriers".
On the other hand, since a male only has one X chromosome, if it is mutated he has no normal copy and will develop the disorder represented by the genetic defect.
If a female carrier has children, there is a 50/50 chance that she will pass on the defective gene to her children. If that child happens to be a girl and inherits the gene, she too will become a carrier.
If the child is a boy and inherits the defective gene, he will develop the disease.
Pyruvate Dehydrogenase Deficiency (E1alpha) is inherited this way.