Here's a good overview with pictures:http://en.wikipedia.org/wiki/Secure_Digital
They differ in:
- Capacity (SD, SDHC, SDXC)
- Read/write speeds (Class 2, 4, 6, 8, 10, etc.)
- Size/shape (standard, mini, micro)
SD is basically an old format and SDXC isn't very common. SDHC is the most common and usable of all the SD formats now. Mini and Micro refer to the physical size/shape of the SD card. The micro's are what you typically see in cell phones, small cameras, etc. Many of...