The show detail removes the table view and replaces the whole screen.
For example on a master Detail project on an iphone it would move from the table view onto the detail view and takes up the full screen, where as on the ipad it would still show the table view as well as the detail view. These two new segues force one way or the other.
The show detail will present the detail view as the only viewable view were as the normal show segue will work like before.