Universally used across a spectrum of digital hardware buying the right SD card can be confusing. The three main considerations when purchasing are:
A) Which Size?
Well you will of course need to check your device but basically there are three physical sizes. The original SDSC standard size of 32 x 24mm and it has nine copper pins. These are most likely to be found in digital photography equipment . Next there is the smaller (and mostly obsolete) miniSD which measures 21.5 x 20mm with 11 pins which will be found in older model smart-phones, or the smallest and now most common microSD which comes in at 11 x 15mm and has eight pins. This size is now the most widely used as it is used in the latest smart-phones. You can however, by using an adapter, use the smaller cards in a standard SD card slot – but it doesn’t work the other way round – you can’t use a standard sized card in a device originally designed to take a smaller card.
B) Size Matters
How Much Storage do I need? The first cards were standard SD with a small 1-2GB capacity. The newer SDHC – Secure Digital Standard Capacity cards depending of course on how much you are willing to spend, can store anything from 2 -32GB. There are also SDXC – Secure Digital eXtended Capacity cards but these have a different file format and cannot be read by older devices with a standard SDHC slot. (SDHC cards use FAT32 file unlike SDXC which use the exFAT file format). Your device will identify which cards are compatible – if in doubt contact the retail outlet where you bought it.
This is probably the most confusing bit and the SD Card Association has created a set of logos which manufacturers have to use to identify the different classes of cards.
Standard class cards have a number displayed within the letter C such as 2 being a minimum of 2MBps, 4 being 4MBps,6 being 6MBps, and 10 for 10MBps (bear in mind this is a minimum – these often run much faster and these cards will also generally be marked with the faster speed in large letters) Again, if you refer to the manual and verify your devices specifications it should tell you the speed at which it writes data to an SD card – match this to your cards and everything will be fine.
Ultra High Speed cards are also available for high end digital photography equipment.
For more you can link here to the Wikipedia entry for SD cards https://en.wikipedia.org/wiki/Secure_Digital