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This is also a conditional event and only fires on postback. This event loads the data of the control, which implements IpostBackDataHandler from the HTTP Post data. Textbox is a good example of such control. Hence these controls will have data even if there view state is disabled. For a control like dropdown list, which gets it data at runtime only the selected value, is retained. Other values of the collection are not retained. 

Page_Load()

This is the most commonly know event of the Page. One thing to remember for this event is that the load event of the other user control is fired after the load method of the page. We can use the IsPostBack property to get whether this is the first load of the page or the page is posted back and add code to be executed only on the first load of the page. Control Event Handlers

This is also a conditional event and fires only on post back. If a user clicks on a button then the button_click is fired. Many a developer have confusion that the when a button is clicked only the click event is fired or page load event is fired after the click event is fired. Many controls (Like data grid) can emit multiple control events.

PreRender()

This event is the last chance to make any changes to the values of the control. This event will fire for all the child controls of the page recursively.

SaveViewState()

As the name suggest, in this event the ViewState is stored in the forms hidden

control. Render()

All the controls are rendered in this event. Again this event is called recursively for each control.

Unload()

Here you can have the page and controls perform clean-up operations. This event has no relevance besides clean up operations because the Page has already rendered. You can also not use any of the values present in the page in this event. 

Hope this helps
Thanks
Vikram