Private members are used to encapsulate the inner workings of your class. Use them to hold data that only you want to be able to access. For example, let's say you have a field called _name and a getter/setter named GetName()/SetName(name). Maybe you want to do some syntax checking on name before you allow the SetName to succeed, else you throw an exception. By making _name private, you ensure that this syntax checking will occur before any changes to name can occur (unless you yourself change _name in your own class, in your own code). By making it protected, you're saying to any potential future inheritor of your class, "go ahead and monkey with my field."
In general, protected is used sparingly and only in specialized cases. For example, you might have a protected constructor that exposes some additional construction functionality to child classes.
