At our first ever Photographer’s Gardener’s Question Time, one of the questions asked was “What do all of those numbers on SD cards mean?“. Andy answered the question by saying that the numbers fell in to 3 main categories;
How big the memory card is, which in turn indicates how many images, or how much video. It’s not possible to say how exactly many images a card will hold since it’s dependant on many things; the camera, the format RAW Vs JPG and even the content of the images themselves. However we know that a larger capacity will mean more images.
As a rule Andy suggested that several small cards are safer than one big one, simply because if a card fails, you only even loose some of your photos.
These settings determine how the memory is card is formatted internally and effectively the maximum size. You need to ensure that if you use a high capacity format like SDXC or SDUC your camera can support that format
The maximum and typical read and write speeds for your memory card are important, since your camera will slow down if it needs to wait for the images to finish being written to the memory card. This especially important if you shoot action photography in RAW in continuous mode, or if you shoot High Definition or 4K video.
Finally Andy suggested that in most cases the best option was to buy a well known brand name (e.g. SanDisk or Lexar) and to choose the range second from top. So in San Disk terms the ranges go Ultra, Extreme, Extreme Pro.
If you’d like to know more about memory cards there’s an excellent article here from Digital Picture.