Plugs and sockets for portable appliances started becoming available in the 1880s, to replace connections to light sockets with easier to use wall-mounted outlets. A proliferation of types developed to address the issues of convenience and protection from electric shock. Today there are approximately 20 types in common use around the world, and many obsolete socket types are still found in older buildings. Co-ordination of technical standards has allowed some types of plugs to be used over wide regions to facilitate trade in electrical appliances, and for the convenience of travellers and consumers of imported electrical goods. Some multi-standard sockets allow use of several different types of plugs; improvised or unapproved adaptors between incompatible sockets and plugs may not provide the full safety and performance of an approved socket and plug combination.

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The International Electrotechnical Commission publishes IEC 60050, the International Electrotechnical Vocabulary, which is also available as IEV Online.

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Generally the plug is the movable connector attached to an electrically operated device's mains cable, and the socket is fixed on equipment or a building structure and connected to an energised electrical circuit. The plug has protruding pins or, in US terminology, blades (referred to as male) that fit into matching slots or holes (called female) in the sockets. A plug is defined in IEC 60050 as an accessory having pins designed to engage with the contacts of a socket-outlet, also incorporating means for the electrical connection and mechanical retention of flexible cables or cords; a plug does not contain components which modify the electrical output from the electrical input (except where a switch and/or fuse is provided as a means of disconnecting the output from input). There is an erroneous tendency to refer to power conversion devices with incorporated plug pins as plugs, but IEC 60050 refers to these as 'direct plug-in equipment', defined as equipment in which the mains plug forms an integral part of the equipment enclosure so that the equipment is supported by the mains socket-outlet. In this article, the term 'plug' is used in the sense defined by IEC 60050. Sockets are designed to prevent exposure of bare energised contacts. Sockets may also have protruding exposed contacts, but these are used exclusively for grounding (earthing).

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To reduce the risk of users accidentally touching energised conductors and thereby experiencing electric shock, plug and socket systems often incorporate safety features in addition to the recessed slots or holes of the energised socket. These may include plugs with insulated sleeves, recessed sockets, sockets with blocking shutters, and sockets designed to accept only compatible plugs inserted in the correct orientation.

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The term plug is in general and technical use in all forms of English, common alternatives being power plug, electric plug, and (in the UK) plug top. The normal technical term (in both British and International English) for an AC power socket is socket-outlet, but in non-technical contexts a number of other terms are in common use. In British English the general term is socket, but there are numerous common alternatives, including power point, plug socket, wall socket, and wall plug. In American English receptacle (which is a term not defined in IEC 60050) is used in official standards and codes such as the NEMA connector standard and the National Electrical Code. The American National Electrical Contractors Association's National Electrical Installation Standards (NECA 130-2010) defines outlet as "A point on the wiring system at which current is taken to supply utilisation equipment." In common use outlet is used to refer to the actual receptacle or socket, sometimes with qualifiers such as wall outlet, electrical outlet and electrical receptacle, all of these sometimes to be found in the same document. There is also colloquial use of socket in American English.

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A socket may be surrounded by a decorative or protective cover called a wall plate, face plate, outlet cover, socket cover, or wall cover. In some designs this is an integral piece with the socket itself, bought and installed as a single unit.

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Electrical sockets for single phase domestic, commercial and light industrial purposes generally provide either two or three electrical connections to the supply conductors. Two-pin sockets normally provide neutral and line connections, both of which carry current and are defined as live parts. Neutral is usually very near to earth potential, usually being earthed either at the distribution board or at the substation. Line (also known as phase or hot, and commonly, but technically incorrectly, as live) carries the full supply voltage relative to the neutral (and to earth). Three-pin sockets provide, in addition, a protective earth connection for exposed metal parts of an appliance. If internal insulation should fail, a short-circuit to the earthed exposed metal parts will hold them at a low potential, and should operate fuses or circuit breakers to isolate the faulty appliance from the supply. Depending on the supply system, some sockets may have two line connections, each at significant voltage to earth and without a neutral pin; for example, a split phase system may have 240 V between line connections each at 120 V with respect to earth ground; but a single-phase receptacle connected to a three-phase system may have, for example, 208 V between contacts and only 120 V between each contact and earth ground.

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An adaptor (in the context of plugs and sockets) is defined in IEC 60050 as a portable accessory constructed as an integral unit incorporating both a plug portion and one or more socket-outlet portions. (There is an alternative spelling, adapter, but adaptor is the form usually used in standards and official documents.)