X-linked recessive genes are genes which are found only on the X chromosome. In humans (although not in all organisms) males are heterogametes, and have one X and one Y chromosome. This means that if the X chromosome carries the recessive allele it will be shown in the phenotype. Females are homogametic, so they may show the recessive phenotype or not. Females will show the recessive phenotype if they are homozygotes, and will express the wild type phenotype if they are heterozygotes because the wild type allele is dominant to the recessive allele.
It is worth noting that the inheritance pattern for X-linked genes differs from the traditional mendelian pattern, for example a female carrier with an unaffected male would give offspring where half the female offspring were carriers, half the female offspring were unaffected, half the males where affected and half the maes were unaffected (theoretically). Because the mother could pass on an X chromosome with the recessive allele or with the wild type allele, and the father could only pass on an X chromosome with the wild type allele.