The terms adjoining and connecting are often used interchangeably to mean the same thing. That is, two hotel rooms that have a connecting door, where guests can move freely between both rooms without using the front door. For family travellers, this allows parents to have more comfort and privacy, yet remain within visible reach of their children comfortably in their own beds next door.
The more accurate term for this is connecting rooms. Adjoining rooms are in fact simply two hotel rooms located next to each other.