A store account does not contain any contact information and is not associated with any one family. It is used to record sales to people who do not have an account with you. For example, someone comes into your Pro Shop to purchase a water bottle who is not one of your students/parents. Instead of having to set up a Family account for this person you can record the sale in a general (store) account.
Watch a (1:34) video tutorial on Store & Family Accounts