The term SD Card covers all types and capacities of Secure Digital Cards, however 'SD Cards' specifically are now only available in 1GB & 2GB.
Secure Digital SD Card
Secure Digital Cards, SD cards or Secure Data Cards were introduced in late 2001 by Toshiba and co-developed with Panasonic..
The Sd card (Secure Digital) is a second-generation derivative of the MultiMedia Card (MMC) standard that is backward compatible with MultiMediaCards (MMC). Improvements over MMC cards include increased in data transfer rates and a physical write protect switch. The SD card is part of a family of SD Card types, that include: Micro SD, Micro SDHC and Mini SD, these cards are all approved by the SD Card Association.
The Secure Digital Card, SD card was designed primarily for use in PDAs and Digital Cameras, the SD card (Secure Digital Memory) is now manufactured by SanDisk, Transcend, Samsung, Lexar, Panasonic, Toshiba & Kingston. Dimensions: 24mm x 32mm x 2.1mm. Available in sizes 512MB, 1GB, 2GB & 4GB
Higher capacity SD cards of 4GB & above in capacity are known as an SDHC Card, this format has a CLASS rating for minimum read & write speed and is not backward compatible with standard SD Card or Secure Digital devices.