SD refers to a type of memory card. The SD is an acronym for “Secure Digital”. SD cards are small and now come in a variety of formats. You can get Micro-SD cards for your phone which are very small, but come with a full SD card sized adapter.
There are many differences between cards, such as speed (Class 2 @ 2mb/s vs Class 10 @10mb/s) and capacity 512mb to 128gb. There are also designations for ‘bus speed’ such as UHS-I, UHS-II and UHS-III with each higher number boasting better performance specs.