A memory card is a physically small storage device that is used in consumer electronics - especially digital cameras - to hold data, usually until it can be moved to a computer. SD, xD, MemoryStick, CompactFlash are examples.
For questions that involve the use of a memory card in some relevant or important way. If the question involves reading it, also tag it with the "card-reader" tag.
Memory cards are usually smaller, both physically and by capacity than USB flash drives and are considered specialized, since many computers do not have the card readers for them. This is compared to USB flash drives and external hard drives, which are useable on almost all modern computer systems.
Currently there is something of a format war going on between the various standards: CompactFlash (CF), xD, SecureDigital (SD), MemoryStick (MS)