Darjeeling is the name for tea grown in the Darjeeling area of West Bengal, India.
The term is more a geographic designation of where the tea is from, than the type. The tea leaves are usually from the Camellia sinensis var. sinensis bush, which has small leaves, but a grower might decide to try another bush. The leaves are usually processed to make a black tea, but they could be processed into green or even white tea.
The tea has to be both grown and processed there. 
87 different “gardens” grow the tea, for 56 different estates as of 2007 
The first first harvest is in March; the second in May and June; third in the summer and fourth in the fall. From December to February, there is no harvest.
Genuine Darjeeling is expensive, so much fake tea that isn’t really Darjeeling is of course sold as such.
Tea bush planting in the Darjeeling area was started by a Dr. Campbell in 1841. Real ones will carry a green certification mark which is a round logo, showing a woman holding tea leaves. This certification mark was created in 1983.
Fans recommend drinking Darjeeling without sugar, either black or with a bit of milk.
 “is cultivated, grown or produced in the 87 tea gardens in the defined geographic areas and which have been registered with the Tea Board; • has been cultivated, grown or produced in one of the said 87 tea gardens; • has been processed and manufactured in a factory located in the defined geographic area;” — Tea Board of India. About Darjeeling Tea. Retrieved September 2010 from http://www.teaboard.gov.in/inner1.asp?param_link_id=610&mem_link_name=About%20Darjeeling%20Tea
 Tea Board of India. Darjeeling Tea Estates. Retrieved September 2010 from http://www.teaboard.gov.in/pdf/DarjeelingTeaProducers.pdf
Gross, Matt. High Tea, India Style. New York Times. 14 October 2007.