The SD card is a downward compatible development of the multimedia card. In 2001, this was developed by SanDisk, based on the older MMC standard. The protocols of both are closely related.

Also, the SD card has an integrated controller. In addition this has a small slide, with which the write protection can be turned on and off. However, this is no protection mechanism at the card, but is only recognized by the specific reader and evaluated by the software.
The SD card has two more pins, which are for data transmission.

Up to 100 Mbps of data can be tranferred in theory, in a stroke of 25 MHz and 4-bit parallel. The actual transmission rate depends on the abilities of the controller in the terminal.
The standard for SD cards defines only one card capacity up to maximum 1 GB (SD 1.0) and 2 GB (SD 1.1). Cards with a capacity of 2 GB, which are sold as SD cards (not SDHC) do not comply with the specifications, and lead especially in older devices sometimes to problems. Furthermore, they do not work in some SDHC devices (e.g. in some digital camera models).
The input / output mode (I/O) of the Multi-Media card has been taken over and is named as SDIO (Secure Digital Input Output) in product descriptions, such as for bluetooth, wireless and other devices.
The SD memory card is built more robust as the multimedia card.


SDHC - SD High Capacity (SD 2.0)

At 512 Mbytes is a limit. There are devices which can only read SD cards up to 512 Mbytes.
An extension (2006) of this standard makes storage capacities up to 32 GB possible under the name SDHC (SD High Capacity). Under this standard, the FAT32 file system can be used. SDHC cards work only in devices, which have implemented the SD specification version 2.0. Meanwhile, there are cards with 8 or even 16 GB. It is recommended, especially for older devices, to examine whether these correspond to the SD 2.0 specification.
The SDHC specification defines performance classes, which guarantee the minimum transfer rates for the recording of MPEG data streams. The speed classification is imprinted from a storage capacity of 2 GB. CLASS 2 is for a minimum of 2 Mbytes/s, CLASS 4 for at least 4 Mbytes/s and CLASS 6 for a minimum of 6 Mbytes/s.


MiniSD

The miniSD card is a minimized SD card, which was specifically developed for mobile phones. This memory card format is completely electrically compatible to the SD card. Security features, and interface are identical. MiniSD cards can be used in an SD-compatible device with an adaptor.


MicroSD

MicroSD was originally developed by SanDisk and initially under the name T-Flash, later under TransFlash. In July 2005, the SD Card Association (SDA) put this memory card under its supervision, and was renamed in microSD.
This memory card format is completely electrically compatible to the SD card. Therefore, a microSD card can be used also as SD-card with a corresponding adaptor.

General technical data from the SD Card series:

Size (WxHxD) in mm:    
SD / SDHC 32 x 24 x 2.1
MiniSD 20 x 21.5 x 1.4
MicroSD 11 x 15 x 0.7

Connecting pins: 8 (9)
Realized capacity: 32MB to 16GB (up to 32GB designed / planned)
Speeds:
read: 3.6 MB / s to about 18MB / s
write: from 0.8 to approximately 20MB / s
Operating voltage: 1.8 to 3.3 volts
Operating temperature range: 0 ° C to 50 ° C.
Resting/storage temperature range: -15 ° C to 65 ° C.
Writing/delete cycles: MLC NAND chips 100,000, SLC NAND chips 1,000,000