taken from Fredrik's blog:
The FormView has support for paging so we don’t really need to add button controls for those commands. We only need to use those if we want to create our own custom paging.
For changing modes:
The New command will change the mode of the FormView to the Insert mode. The Cancel will turn back the mode to its default mode. The Edit will turn the mode into Edit mode.
When we press a button with the Insert command specified the FormView will execute the associated DataSource control’s InsertCommand. If the CommandName of the button is set to Delete or Update the DataSource’s DeleteCommand or UpdateCommand will be executed. By default the SelectCommand of the DataSource control will be executed.