SD cards come in three physical sizes, tiny, tinier and tiniest. The standard cards, about half the size of a CompactFlash card, are 24 x 32 x 2.1mm. Mini SD cards are 21.5 x 20 x 1.4 mm. Micro SD cards are 15 x11 x 1 mm. Originally these cards were designed to hold a maximum of 2 Gb. Oh designers, what were you thinking? I can burn 2 Gb of photos on my way to the plane to go on vacation!
Enter the newer SDHC standard from the SD Card Association. To increase capacity above 2 Gb the file system was changed from FAT16 to FAT32. Unfortunately, older card readers are stuck on the original system. Put an SDHC card in an old-style standard reader and it might as well be speaking Yiddish.