mdi chile form in vb.net

MDI is like a Parent Form

This MDI form used to open many child form (i.e. normal window form) inside MDI form

The Question in not completely incorrect, but can be greatly improved. You should have re-formulated it:

"What are Wikipedia, Google, CodeProject and StackOverflow.com?"

If you asked and got good Answers, the Questions like your original one would not be needed!

Now, about MDI: I would strongly recommend you never user it!
See some motivation:
http://en.wikipedia.org/wiki/Multiple_document_interface#Disadvantages[^],
Question on using MDI windows in WPF[^],
MDIContainer giving error[^].

First of the links above will also answer your question perfectly.

—SA