There are two basic classifications of NEMA device: straight-blade and locking. The straight-blade 5-15 and 5-20 are found nearly everywhere in countries using the NEMA standards, and are intended for supplying lighter-duty, general-purpose electrical devices. Twist-locking types are used for heavy industrial and commercial equipment, where increased protection against accidental disconnection is required. Numbers prefixed by "L" are curved-blade, twist-locking connectors; others are straight blade and non-locking. The metal conductive blades are sometimes informally called "prongs" (as in "3-prong plug").

The numeral preceding the hyphen in NEMA nomenclature indicates the configuration, that is, the number of poles, number of wires, voltage, and whether single- or three-phase. A grounding type of device is described as two-pole, three-wire; or four-pole, five-wire; etc. A non-grounding device may be two-pole, two-wire; or three-pole, three-wire; etc.

The numeral following the hyphen is the rating of the device in amperes. The number is followed by the letter R to indicate a receptacle (female connector) or the letter P to indicate a plug (male connector).

As an example, the 5-15R is the common 125 V two-pole, three-wire receptacle. The L5-15R, while sharing the same electrical rating, is a locking design that is not physically compatible with the straight-blade 5-15 design. The 5-30 has the same two-pole, three-wire configuration and 125 V rating, but is rated 30 A.

Although there are several non-grounding device types in the NEMA standards, only three of them are in widespread use today. These are the two-pole 1-15, still in use in millions of buildings built before the 1960s, and the three-pole 10-30 and 10-50.

The small hole near the end of the power (non-ground) blades of some NEMA plugs is used for convenience in manufacturing; if present, it must be of specified diameter and position.Small specialized padlocks are available to fit these holes, allowing "lockout" of hazardous equipment, by physically preventing insertion of locked plugs into a power receptacle.

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