Secure digital (SD) cards started out as a wonderfully simple Flash storage format, and quickly replaced CompactFlash, MMC and other types of card. But as usage grew, people began to need different speeds and sizes. The result is the chaos we enjoy today.
For standard applications like single-shot compact cameras, it doesn't matter which class of SD card you buy, as long as it's compatible. Storing a 500K to 2MB file is not hard. However, if you want to shoot a rapid sequence of photos, the card has to be fast enough to keep up. If you want to shoot high-definition videos, keeping up can be a challenge. For this sort of application, you'll probably need a fast card.
When buying an SD card, you have to consider three things: the physical size, the storage capacity, and the speed at which it can write data.
There are now three sizes of SD card. The original format started with cards measuring 32 x 24mm, which was very small for the time. These are still common in digital cameras, audio recorders and similar products. Smartphone makers wanted smaller cards. This resulted in the miniSD format, measuring 21.5 x 20mm, and then microSD cards, measuring 11 x 15mm.
You can use a miniSD or microSD card in an SD card slot by plugging it into an SD-sized adapter, and some mini/micro cards are sold with them. Obviously, you can't fit an SD card into a miniSD or microSD slot.
I assumed most of us needed different sizes, so it's handy if all your devices use the microSD format. However, other things being equal, full-size SD cards are generally faster than small ones.
When it comes to storage, there are three different types of card: SD, SDHC (High Capacity) and SDXC (eXtended Capacity).
Originally, an SD card could hold up to 2GB of data using Microsoft's FAT-12 or FAT-16 file format. This was a huge amount in 1999, and most cards stored a lot less. In 2006, when the 2GB limit was becoming a problem, the industry introduced high capacity SDHC cards that could store up to 32GB using FAT-32. After that, in 2009, came SDXC cards able to store up to 2TB using Microsoft's proprietary exFAT (Extended File Allocation Table) format.
Apple started supporting exFAT with Mac OS X 10.6.5 (Snow Leopard), which makes exFAT the best choice for shared hard drives or transferring very large data files. It also means that future SD cards will be able to use exFAT up to 512TB and, in theory, beyond. Either way, the SD Card Association provides a free SD Formatter 4.0 for SD/SDHC/SDXC program for Windows and Mac OS X for people who want to check or reformat cards. This optimises performance and protects the Secure Digital card's secure "Protected Area".
Today, 8GB and 16GB cards are big enough for most purposes, though people may well choose a 32GB or 64GB card to expand the storage in an MP3 player or mobile phone. If buying a 64GB or 128GB card, it's very important to check that your device can support it. The Samsung Galaxy S4 can handle 64GB cards, but most phones can't, and I don't know of a European or American phone that supports 128GB cards. If you need 1TB or 2TB of storage, you'll have to buy an external hard drive.
Things get confusing when it comes to speed ratings. The SD Card Association has different classes of cards, and produced a bunch of mandatory logos with the class number inside a big C. However, Class 2 is 2MBps, Class 4 is 4MBps, Class 6 is 6MBps, and Class 10 is 10MBps or faster -- sometimes much faster. For this reason, faster cards are often marked with the speed in large letters, such as 45MB/s or 90MB/s.
If you check the specifications of your devices, you should be able to find out how fast it can write data to an SD card. As long as your cards can handle that data rate, you should be OK.
Note that the class rating shows a minimum speed, not the actual speed. A good Class 2 card may work faster than a Class 6 or even a Class 10 card. The only way to find the real performance of a card is to run a benchmark test.
Also bear in mind that megabytes per second -- MBps, usually written as MB/s on cards -- are eight times faster than megabits per second (Mbps), because there are 8 bits in a byte. A 2MBps card is a 16Mbps card.
Finally, SD card speeds are sometimes cited as "100x Speed" or whatever. This is somewhat akin to measuring things in football fields or areas the size of Wales. It's based on the speed of an original CD-Rom drive: 150 KBps. You will recall that you ended up with a 16x or 48x CD-Rom drive, and you can extend the idea to 48x or 400x SD cards. I don't find this a particularly useful way of rating SD cards, unless your camera manufacturer specifies 50x or 100x cards, or whatever.
For most purposes, you can buy any card that your device supports, and ignore the speed ratings. For example, your camera's spec will say that it takes microSD, microSDHC or microSDXC cards, so just buy whichever type it says. Other things being similar or equal, buy whichever card is faster.
Not many devices have UHS-I buses at the moment, and usually they are more advanced models. For example, looking at Canon DSLRs, the EOS 500D takes SDHC cards, the 600D can use SDXC cards, and the 650D is the first model that can handle UHS-1 or U1 cards. (The EOS M also sports UHS-1, as do the Nikon D5100 and D7000.)
In general, I'd avoid buying UHS-1 cards unless your device specifies them, because the new bus has new pins and reassigns some old pins. Cards with both logos – ie C10 and U1 – may work in most (but not all) other devices, but will usually cost more. They still won't write data faster than your camera can send it.
You could argue that UHS-1 will become common and that people will reap the benefit when they buy new cameras, or whatever. That might be true for people who are planning to buy more advanced devices fairly soon. The rest of us can wait until UHS buses filter down to cheaper devices, by which time, UHS cards will be cheaper.
I'm not against spending money on SD cards, but I think it's better to spend it on respected name-brand cards and not on cheap ones. As was the case with CD-R discs, not all Flash memory chips are created equal. Personally, I use SanDisk and Kingston cards, but Toshiba, Panasonic, and Lexar Pro and Platinum II cards also have good reputations. Panasonic invented the SD format, and developed it with SanDisk and Toshiba.
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