In the Nonprofit Success Pack there are two basic types of accounts: Household Accounts and Organization Accounts. Though there may be multiple account record types for organization accounts (such as Media, Nonprofit, etc.), they are all essentially Organization accounts.
Household Accounts are only used to represent a grouping of people by household. Contacts are placed directly "on" the Household Account (their Account Name is the Household Account.)
Organization accounts may be used for many purposes. Primarily, of course, they represent organizations that CAC works with, grantmaking philanthropies, etc.
The "Internal" record type is used for accounts that are created for the purpose of grouping people without being an actual "organization" in the world, such as the Clean Air Council Board of Directors account or any other boards/committees/commissions of CAC.
Contacts should never be placed directly "on" an organization account--they should have an Affiliation to the account, whether their Primary Affiliation or another affiliation record. Similarly, Primary Affiliation should never be to a household account. There are validation rules in place to prevent saving records in either of these situations.