NEMA classifications of protection

The National Electrical Manufacturers Association (NEMA) is a US manufacturers organization which actively promotes standardized product specifications for electrical apparatus.
NEMA performance criteria and test methods are used by Underwriter Laboratories (UL) as guidelines for investigation and listing of electrical enclosures.

NEMA  Description
1Indoor use, 
primarily to provide a degree of protection against contact with the enclosed equipment and against a limited amount of falling dirt.
(~ IP 30)
2Indoor use,
to provide a degree of protection against limited amount of falling water and dirt.
(~ IP 31)
3Outdoor use,
to provide a degree of protection against windblown dust, rain and sleet; no damage caused by formation of ice on the enclosure.
(~ IP 64)
3ROutdoor use,
to provide a degree of protection against falling rain and sleet; no damage caused by formation of ice on the enclosure.
(~ IP 32)
3SOutdoor use,
to provide a degree of protection against windblown dust, rain and sleet; external mechanism remains operable while covered with ice.
4Indoor or outdoor use,
to provide a degree of protection against splashing water, windblown dust and rain, hose directed water; no damage caused by formation of ice on the enclosure.
(~ IP 66)
4XIndoor or outdoor use,
to provide a degree of protection against splashing water, windblown dust and rain, hose directed water and corrosion; no damage caused by formation of ice on the enclosure.
(~ IP 66)
6Indoor or outdoor use,
to provide a degree of protection against the entry of water during temporary submersion at a limited depth; no damage caused by formation of ice on the enclosure.
6PIndoor or outdoor use,
to provide a degree of protection against the entry of water during prolonged submersion at a limited depth; no damage caused by formation of ice on the enclosure.
11Indoor use,
to provide a degree of protection during oil immersion against corrosive effects of corroding liquids and gas.
12,12KIndoor use,
to provide a degree of protection against dust, falling dirt and dripping non-corrosive liquids.
(~ IP 65)
13Indoor use,
to provide a degree of protection against dust, spraying water, oil and non-corrosive coolants.
(~ IP 65)

(Source reference: Fibox GmbH)