Break-away Tabs
Duplex receptacles have break-away tabs to separate the top and bottom outlets, so that they can be placed on separate circuits. This may allow for one switched outlet for a lamp, or for two circuits when heavy loads are anticipated. Two branch circuits may share a neutral terminating on duplex receptacles, a condition sometimes referred to as "split-wiring", "split-receptacle", or "half-split".
Neglecting to break the tabs when replacing a split receptacle can disable a switched outlet, or cause a short circuit, or accidentally parallel two circuits, depending on how the receptacle had been wired.
Read more about this topic: NEMA Connector