What is state? What is the difference between states and ViewStack?

View Stack is to handle different MXML file e.g. TAB control. And states is the transition within single MXML file ViewStack should be used where there is complete change in the controls used and States should be used when we just want to add or remove a few components based on certain conditions. Login/Registration/Forgot password is the best example for using States as each page will either add or remove to the already existing one.