Understanding SD Cards
What do the symbols on a Micro SD Card mean?
This is the type of card; different card types use different file formats and newer cards won’t work in older card readers.
microSD: Has a capacity up to 2GB, and works in any microSD slot.
microSDHC: Has a capacity of more than 2GB and up to 32GB, and works in hardware that supports either SDHC and SDXC.
microSDXC: Has a capacity of more than 32GB and up to 2TB (although at the time of writing, 1TB is the largest available card), and is only supported in SDXC-compatible devices.
microSDUC: Supports cards up to 128TB, and will require a compatible device.
3. UHS Speed Class Rating
This is the minimum sustained writing speed of the card; important for video recording. UHS Speed class 3 cards will never write slower than 30 MB/s, UHS Speed class 1 cards never slower than 10 MB/s.
4. Speed-Class Rating
This is an older speed-class rating. It is redundant of the UHS speed class, but many card manufacturers include it, as well, since many consumer products still recommend products based on the old standard. A Class 10 is the fastest of the old speed class ratings and a class 10 card is verified to never write slower than 10 MB/s, class 4 would never be slower than 4 MB/s.
Here's a good reference showing what you can record on an SD card based on size.
At 14MP you can take 26000 photos on a 128GB card
At 4K you can take 4:40
At 1080P you can take 9:20
6. Bus Speed Interface
We hope that gives you a good overview of how to decipher SD cards.