Create a Sim (also known as CAS) is the part of Create a Family where individual Sims are made. It is used to customize a Sim's appearance and personality. As a rule, only superficial changes can be made to a Sim's appearance once he or she is added to the game. However, there are exceptions to this.
Create a Family is also used to organize Sims into families or households, as well as customize the relationships within a family or household before adding it to a neighborhood. However, since Create a Sim is the main part of Create a Family, players tend to refer to "Create a Sim" when talking about Create a Family.