Get value from popup window to parent window

I have a website which has a popup window (this only opens when the user
chooses to open it). In the popup window I have a control which
lists a selection of "classes". Each class has a description and a class_id
(stored in the value attribute of each option). The user will then select a
class from the drop-down list.

What I want to do is have a control in the parent browser window which can
store the class_id and the description that the user has selected in the
popup window.

Any suggestions as to what control I should use in the parent window and
more importantly how I get the value from the popup window (this must be
client side code as the user will have entered other values into the form in
the parent browser window)?

TIA