basic settings from the default display which all Views must have. For most settings, there is an override button that will override that single setting for the current display. Overridden settings will have a mark in the summary for that display. All 'default display settings' are shown in the other displays in 'italic'. When you override a setting, then it is shown 'normal'.
Please keep in mind that when you are editing a setting on a display that is not overridden, then by default you are editing that for all displays.
Overriding fields, arguments, sorts, filters and relationships, can only be done by overriding the entire group or none of them. To do this, click on the header for the filters or the rearrange button. Once you override, the display will then have its own copies of the fields/filters/etc and changes to the defaults will not be reflected on your display.
With the Reorder button you can organize the order of your displays.
With the Analysis button the system checks the view and may give you suggestions if something is wrong.
You can clone a display by using the link in the header of the display.