So, what's a carrier of a genetic disease, then? Well, in the case of a recessive single-gene disease, it's someone who is heterozygotic for that gene, ie they have one dominant, healthy allele, and one recessive, disease-causing allele. As with Mendel and his pea plants, if two "carriers" have children, one in four of them will have two recessive alleles and hence the disease.
Simple, right? Actually, no. You'll be able to work out from what I've already told you that a carrier of a disease will be expressing the diseased protein as well as the healthy one, in roughly equal amounts. These individuals do not have the disease because, in most cases, either half the amount of healthy protein is enough, or the body will detect that it doesn't have enough and upregulate production of that protein. Which will, of course, also upregulate production of the diseased version, which will hang around the cell doing nothing until it is tagged by the recycling machinery and broken up to have its parts reused.
So, you'll be unsurprised to here that there are instances in which there *are* effects from having one healthy and one diseased copy of a particular gene, the textbook example here being sickle-cell anaemia. Patients with this condition have a mutation in haemoglobin, the protein that carries oxygen in the blood, which causes the haemoglobin molecules to clump together and bend the red blood cell into a curved shape which not only is less efficient at carrying oxygen, but has more trouble fitting down the narrowest capillaries to provide that oxygen to the tissues that need it.
People with two faulty copies of this gene have the predictable symptoms of being unable to oxygenate their body properly, and a higher potential for organ problems caused by the capillaries supplying blood being blocked. One copy "just" leaves you needing to be careful if you take a lot of exercise or go somewhere with a low oxygen level like up a mountain, or if you have other blood-related conditions as well, which is only to be expected as half your haemoglobin is faulty.
So, why is sickle cell anaemia so prevalent? There are areas of Africa in which 25% of the population have either one or two copies of the faulty gene, despite the selection pressure against having sickle cell anaemia. The answer lies in the prevalence of malaria in the same areas- heterozygotes for sickle cell disease have blood cells that are harder for the Plasmodium parasite that causes malaria to infect. SO these people are less likely to die young of malaria, and thus more likely to pass on their malaria-resistance gene that "just happens" to have horrible disadvantages attached...