SD cards up to 2Gb will work in all SD compatible devices, cards above 4Gb are compliant with a new standard called SD High capacity or SDHC for short. SDHC cards will only work in devices showing the following logo. Card readers also need to have the correct logo to work with the cards. High capacity cards are not backward compatible with standard SD devices.
Standard and high speed Ultra and Extreme SD cards are available, the Ultra and Extreme cards can read write data faster than the standard cards, although this can also depend on the device. High capacity cards and devices comply with the following speed class ratings wich specify a minimum sustained write speed for SDHC/miniSDHC/microSDHC cards (Class 2: 2 MB/s; Class 4: 4 MB/s; Class 6: 6 MB/s). look for the following logos to identify your card speed.