What's the basic difference between genotype and phenotype?

An organism’s genotype is the set of genes that it carries. An organism’s phenotype is all of its observable characteristics—which are influenced both by its genotype and by the environment. So in defining evolution, we are really concerned with changes in the genotypes that make up a population from generation to generation. However, since an organism’s genotype generally affects its phenotype, the phenotypes that make up the population are also likely to change.