Show:
Use Show to set the form Visible property to true and to bring the form to the front of other forms on the screen.
ShowModal:
Use ShowModal to show a form as a modal form. A modal form is one where the application can continue to run until the form is closed. Thus, ShowModal does not return until the form closes. When the form closes, it returns the value of the ModalResult property.
翻译成中文大概意思是：
Show仅仅是让一个窗口显示出来，比如在Form1中让Form2->Show()，那么在Form2显示之后你还可以继续点击Form1的东西。
而ShowModal是把窗口当成模式窗口显示，意味着程序只能在这个窗口关闭以后才继续执行下面的语句，比如在Form1中让Form2->ShowModal();那么在Form2关闭之前，Form1是不可用的。