A
carrier, in
genetics, is a
heterozygote. In this case, the
dominant gene (the one that shows) is the normal gene, and the
recessive gene codes for some abnormality, be it
sickle-cell anemia or
color-blindness or
hemophilia or whatever. As such, the person appears normal, but presents a chance of passing on the abnormality to any offspring if he or she produces children with another carrier or an afflicted person.
It's worth mentioning that in terms of X-linked abnormalities in humans, only the female can be a carrier; the male, having only one X chromosome, will automatically show his genotype in phenotype.
It is often recommended that if you know that certain diseases, such as hemophilia or sickle-cell, run in your family, that you get tested to see if you are a carrier, as some of these abnormalities can be devastating.