Memory card features and specs
To start with you’ve got a choice of sizes: The standard SD ones (mostly for digital cameras and bigger gear) and the smaller microSD ones (originally developed for, and still used in, smartphones). Extra letters after the SD mean a newer, improved standard, with room for greater capacities and faster speeds—these include HC (High Capacity) and the latest XC (Extended Capacity), and both are used across the SD and microSD form factors today.
MicroSDXC cards are the top choice for your phone, and the SDXC cards are your top choice for everything else. After that, the key spec is obviously the capacity, or how much data a card can hold. Newer HC and XC cards offer more storage because of their file systems and various other tweaks, with the newest cards on the market now hitting 512GB (with a theoretical limit that’s even higher).
Then there’s speed, or how fast a memory card can read and write data. If you’re shooting high-resolution images, you don’t want to have to wait seconds between shots for them to be written to your memory card. Write speed is most important, and is usually specified with a Speed Class rating. The minimum performances you can expect are Class 2 (at least 2 MB/s), Class 4 (at least 4 MB/s), Class 6 (at least 6 MB/s), or Class 10 (at least 10 MB/s). Some card makers will also list a maximum speed and/or a read speed separately to distinguish between different cards.