It has to be GFCI protected, and a weather-resistant type receptacle must be used. Also, the cover must be weatherproof when nothing is plugged in, though I recommend an "in-use" or "extra-duty" cover.
You shouldn't have any trouble finding a weather-resistant GFCI receptacle, and weatherproof covers for this style of receptacle are also easily found.
Article 210.8(A)(3) of the National Electrical Code, says that outdoor receptacles must be GFCI protected.
National Electrical Code 2014
Chapter 2 Wiring and Protection
Article 210 Branch Circuits
210.8 Ground-fault Circuit-Interrupter Protection for Personnel.
(A) Dwelling Units. All 125-volt, single-phase, 15- and 20-ampere receptacles installed in the locations specified in 210.8(A)(1) through (10) shall have ground-fault circuit-interrupter protection for personnel.
Article 406.9(A), says that receptacles in damp locations must have covers that are weather-proof when nothing is plugged in. While 406.9(B)(1), says that receptacles in wet locations must have covers that are weather-proof even when something is plugged in. Both sections also require the receptacle to be a weather-resistance type receptacle.
Chapter 4 Equipment for General Use
Article 406 Receptacles, Cord Connectors, and Attachment Plugs (Caps)
406.9 Receptacles in Damp and Wet Locations.
(A) Damp Locations. A receptacle installed outdoors in a location protected from weather or in a damp location shall have an enclosure for the receptacle that is weatherproof when the receptacle is covered (attachment plug cap not inserted and receptacle covers closed).
... All 15- and 20-ampere, 125- and 250-volt nonlocking receptacles shall be a listed weather-resistant type.
(B) Wet Locations.
(1) Receptacles of 15 and 20 Amperes in Wet Locations. Receptacles of 15 and 20 amperes, 125 and 250 volts installed in a wet location shall have an enclosure that is weatherproof whether or not the attachment plug cap is inserted. An outlet box hood installed for this purpose shall be listed and shall be identified as "extra-duty".
All 15- and 20-ampere, 125- and 250-volt nonlocking type receptacles shall be listed and so identified as the weather-resistant type.
Personal preference here, but I like the type of "extra-duty" cover that accordions out when in use. Something like the Hubbell-TayMac ML450G.
As it maintains a slimmer profile when not in use, but allows ample space for routing the cord when something is plugged in.