Continuous variation: there is a wide range of phenotypes (e.g. height)
Discontinuous variation: phenotypes fall into discrete categories (e.g. blood type)
Discontinuous variation tends to be coded for by one gene with a few different alleles. However, continuous variation is more complex. This is usually coded for by many genes (polygenes), with many alleles, which produces the much greater range of possible phenotypes.
Polygenes can give rise to susceptibility to disease, usually with an environmental trigger. Diseases that are both genetic and environmental are called multifactorial.