It is simple stuff, and isn't unique to Form-derived classes.  It's basic  
OOP.

That said, here's a simple example (class members unrelated to the  
specific question omitted for brevity):

    class Form1 : Form
    {
        void ShowForm2()
        {
            using (Form2 form2 = new Form2())
            {
                form2.ShowDialog();
                MessageBox.Show(form2.Data);
            }
        }
    }

    class Form2 : Form
    {
        public String Data
        {
            get { return "Form2 Data"; }
        }
    }

If the form's not modal and you need Form1 to keep the reference, then it  
will need to store that reference in a field somewhere.  If Form2 needs a  
reference to Form1, you can provide that when you create the Form2  
instance.  For example, pass the reference of Form1 to the constructor of  
Form2, or set a property on Form2 after it's been instantiated.

Pete