Penetrance denotes the percentage of carriers of a mutation who actually develop the phenotype associated with the mutation. Complete penetrance consequently means that all persons with a particular genotype become ill. Huntington's disease, for example, has a penetrance of 100 % because all persons who inherit an expansion mutation in the gene coding for Huntington's disease become ill with this disease. With the vast majority of genetic diseases, however, the level of penetrance is considerably lower.
Expressivity is a term used to characterise the degree or variance of a genetic trait: it may be expressed strongly, weakly or somewhere in-between. What is at stake here, then, is the trait in relation to the individual person.