SD card, is also called secure digital, is a popular format of memory cards, used in cameras, PDAs, and other devices as well, and most MMC cards can be used as SD cards

Mini SD, is smaller in size, used more in mobile (cell) phones as memory, like Nokia; with the adapter included usually, Mini SD can also be used as SD card, so giving you more options, of course, same cpacity Mini SD is a bit dearer than SD

Micro SD, another format of memory, is fully compatible with Trans Flash (T Flash, T/F), is also smaller size than SD, used more in mobile (cell) phones as memory, like Motorola, Samsung; similar to Mini SD, it can also be used as SD card with the adapter included, so giving you more options, of course, same cpacity Micro SD is a bit dearer than SD.

 

In short, Mini SD, Micro SD can be used as SD=MMC, while SD can NOT be used as Mini/Micro SD.