The quick and simple answer: Secure Digital. Utilizing what’s known as nonvolatile memory, a Secure Digital memory card is very much like a small size USB flash drive, in that, just like a flash drive, a SD card will retain its data even without being plugged into a computer, camera, or other device to power it. The “Secure” in Secure Digital comes from the SD cards’ original goal of providing a form of copyright protection for various media types (primarily music and movies) and their copyright holders, directly on the card. This form of copy protection, known as Digital Rights Management (DRM) allows for the use of special codes to be embedded on an SD card, codes which tell the device using the SD card that it’s ok (or not) to access the music, movies, or other data on the SD card. Despite some initial interest by corporate copyright holders in utilizing the DRM provided by SD cards, this form of copy protection never quite took off, leaving SD cards to become very popular as removable storage for devices where DRM is not typically required, such as digital cameras and cell phones.
Over the years, SD cards have become ubiquitous in many markets as the preferred choice for said cameras, cell phones, and many other devices. As such, SD cards are now available in a multitude of styles, with the most common SD cards broken down into the following types:
SD: The original Secure Digital card form factor. Just slightly larger than a postage stamp, SD cards are easy to pop into and out of cameras and into computer readers, but are limited in capacity to just 2 gigabytes; making them good for storage of smaller files such as low resolution photos or text documents. A bonus feature found on the standard SD card: a write protection switch which can be flipped to make your SD card “read only;” preventing accidental deletion or overwhelming of important files, documents, or photos.
miniSD: A miniature version of the standard size SD card, miniSD cards can also hold up to 2 gigabytes of files, on a card that is physically 37 percent smaller than a “normal” SD card. Does not include a write protection switch, and requires an adapter (typically included with purchase) to transfer data from a miniSD card to other devices via a standard SD card slot.
microSD: Even smaller than the miniSD is the microSD card. At only about a quarter the size of a standard SD card, these super small memory cards commonly serve duty in small form factor applications such as cell phones. Just like the miniSD card, microSD cards are limited in capacity to 2 gigabytes, utilize standard SD sized adapters for some card readers, and also do not include a write protection switch.
SDHC: An acronym for Secure Digital High Capacity, SDHC cards are available in the same sizes and features as the standard, mini, and micro SD cards mentioned above, but in much higher storage capacities; ranging from 4 gigabytes all the way up to 32 gigabytes. This jump in storage capacity gives SDHC cards the ability to hold thousands of photos, hours of high definition video, hundreds of music files, and much, much more. In addition to larger amounts of storage, SDHC cards also transfer data faster than normal SD cards; making them perfect for products which require real time data storage, such as video cameras. To further help consumers decide which SDHC card is best for use in their camera, phone, or other device, SDHC cards come with speed ratings known as “classes.” SDHC classes fall into four categories: Class 2, Class 4, Class 6, and Class 10; the higher the number, the faster the card transfers data. In addition, there is currently one additional speed class: UHS-1. Faster than any other class, UHS-1 capable SDHC cards are particularly useful for professional quality video/audio projects where real time recording of high definition content is a must.
SDXC: Secure Digital Extended Capacity. These bad boys start at a minimum 32 gigabyte storage capacity with plans for SDXC cards which go all the way up to 2 terabytes of storage, with speeds capable of exceeding UHS-1. The SDXC format is still quite new, and as such fewer devises support them. However those that do benefit from the largest storage capacity and fastest data transfer speeds currently available in a SD card.
And that, in a very large nutshell, is what the SD in SD card means.
~ J. Conboy