One or both outlets may be electrically live at all times, or one or both may be controlled by a wall switch. The receptacle may be installed in the middle or at the end of a circuit. The wiring arrangement is different in each case. White or silver screws are neutral.
Green screws are for grounding.
Receptacles may be grounding or non-grounding types. Always replace a receptacle with a grounding type unless there's no grounding wire in the box or the box isn't grounded, then you can use a non-grounding receptacle. To install a grounding receptacle to an undgrounded box, you must ground the receptacle independently. For help, consult a professional. Because receptacles are rated for a specific amperage and voltage, be sure to replace an old one with an exact duplicate.