Genetic association refers to the direct genotype-phenotype relationship.
The use of the term is to differentiate it from genetic linkage, which refers to within-family cosegregation of genotype with phenotype arising due to colocalization on the same chromosome.
It can be detected in conventional epidemiological designs such as a case-control or cohort design, as well as in family based designs, which are complex matched (or counter-matched).
A Genome wide association study is such a study testing for association between a phenotype and a large number of marker polymorphisms.
A candidate gene association study will test for association between a phenotype and polymorphisms within a gene that is likely to be related to that phenotype, based on biochemical or physiological evidence.