"protected" methods in C#?

Often 'protected' is used when you want to have a child class override an otherwise 'private' method.

public class Base {
    public void Api() {
       InternalUtilityMethod();
    }
    protected virtual void InternalUtilityMethod() {
       Console.WriteLine("do Base work");
    }
}

public class Derived : Base {
    protected override void InternalUtilityMethod() {
       Console.WriteLine("do Derived work");
    } 
}

So we have the override behavior we know and love from inheritance, without unnecessarily exposing the InternalUtilityMethod to anyone outside our classes.

var b = new Base();
b.Api();  // returns "do Base work"
var d = new Derived();
d.Api(); // returns "do Derived work"

1. Protected methods can be accessed by inheriting classes where as private methods cannot.
2. Keeping in mind that .aspx and .ascx file inherit from their code behind classes (default.aspx.cs), the protected methods can be accessed from within the .aspx/.ascx

Keep this in mind too: If you have a button and that button's OnClick is set to Button_Click

<asp:Button id="btn" runat="server" OnClick="Button_Click" />

then the Button_Click method needs to have at least protected visibility to be accessible by the button.

You could get around this by added the following to you Page_Load method:

btn.Click += new EventHandler(Button_Click);

Protected methods can be called from derived classes. Private methods can't.

That's the one and only difference between private and protected methods.