The difference between these two is that single-gene traits are traits that are determined by one gene.
Traits are usually complex, meaning that several genes controls the outcome. For complex traits, environment also matters. For example, height is controlled by several genes and the environment. Two persons with identical genes can have different height if one is being malnourished and the other is not.
It is also much harder to locate the genes controlling complex traits, as there is an unknown number of them.
