All Secure Digital (SD) storage devices contain one or two NAND flash memory chips and a processing unit to manage the operation of the card.

There are three sizes of SD Card permitted by the spcification:

1. original : 1.26 x 0.94 x 0.083 to 0.055 inches
2. miniSD : 0.85 x 0.79 x 0.055 inches
3. microSD : 0.56 x 0.43 x 0.039 inches

or, in metric:
1. original : 32 x 24 x 2.1-1.4 mm
2. miniSD : 21.5 x 20 x 1.4 mm
3. microSD : 15 x 11 x 1 mm

Standard SD cards must have with a toggle switch that that permits or blocks write operations on the card.
The mini and micro sized cards don't offer this feature.

SD cards are available in five storage capacities:
1. Standard (SDSC): up to 2 GB
2. High (SDHC): from 2 to 32 GB
3. Extended (SDXC): from 32 GB to 2 TB
4. Ultra (SDUC): from 2 TB to 128 TB
5. Input Output (SDIO): storage with a Bluetooth transciever and/or a GPS receiver

# Storage Formats

* SDSC FAT12, FAT16, and FAT16B.
* SDHC FAT32 
* XC/HC exFAT

The exFAT file format was specifically designed for NAND flash devices and is likely to remain the standard for many more years.