As I can see there is a lot of confusion regarding SD memory cards . So, to let you all know...
We do have 3 main formats of SD cards:
1. Secure Digital (SD)
2. mini Secure Digital (miniSD)
3. micro Secure Digital (microSD)http://www.sandisk.com/Assets/Products/130/MMSD_3in1.jpg
In most cases you can use converters and make smaller card to work as a bigger (for example microSD as a normal SD).
Please keep in mind that MMC cards are also working in SD card slots
Plus now we do have a new SD format (mostly incompatible with SD) called SDHC (Secure Digital High Capacity) and miniSDHC, microSDHC (those last two are not available yet on the market).
To read more about all those memory cards I would recommend wikipedia:http://en.wikipedia.org/wiki/Comparison_of_memory_cardshttp://en.wikipedia.org/wiki/Comparison_of_memory_cards
Hopefully now you will be able to tell the difference between those cards. (And buy the one which you really need!)