What is the difference between HorizontalAlignment and HorizontalContentAlignment in WPF?

What is the difference between:

• HorizontalAlignment="Stretch"
• HorizontalContentAlignment="Stretch"

in a textbox in WPF?

Sample example:

<TextBox HorizontalAlignment="Stretch"
            HorizontalContentAlignment="Stretch"
            Height="100"
            TextWrapping="Wrap"
            AcceptsReturn="True"
         ></TextBox>