MDI is Multi Dimensional Interface. It means through a MDI form we can open multiple forms at the same time,without closing any one of them.You cannot place any control in a MDI form.
multiple document interface.When we want no of forms to be treated as a group then mdi solves this problem by providing one large form, which acts as a container for child forms.all the child forms cannot be moved outside the boundary of this form. characterstics of child form 1)when an mdi childform window is maximized its caption is merged with the mdi parent 2)mdi child form cannot be modal 3)when u minimize a child form it shrinks to an icon within mdi parent client area 4)mdi child forms can be arranged in mdi parent's client area using arrange method of the parent form 5)if a child window has a menu then its menu replaces the parents's menu