Android : What is the different between to reinitialize data of Activity in onCreate() and onRestoreInstanceState() in Android

What is the different between to reinitialize data of Activity in onCreate() and onRestoreInstanceState()

I am getting bundle in oncreate aslo.So My question is why I am not able to set data in oncreate method while the same thing is done in onRestoreInstantstate..Then what is the use of bundle object in oncreate i went through different tutorial but not got relevant answer so please if any one know the difference that where does we use the implementation to reinitialize the data of activity that whether it should be in onCreate or onRestoreInstanceState................please let me know