Speed denotes the read/write speed of the memory cards. Memory cards often come with their transfer speeds (both read and write speeds) mentioned on them. Usually the write speed is the one that is more commonly written. You may find numbers such as 2MB/s or 4MB/s written on the cards. This suggests that the card’s maximum write speed is 2 or 4 megabytes per second. SD, SDHC and SDXC cards nowadays comes with a class rating such as Class 4, Class 6 or even Class 10. This is a standardised classification that usually gives an indication of the minimum write speed that the card is capable of reaching and thereby satisfying the needs of certain types of photography. DSLR users should use a Class 10 SD card because the larger image sizes that DSLRs create, need a card with the fastest write speed possible.
UHS-1 is a new classification standard to denote write speed of SD memory cards. UHS-1 promises a write speed of more than 70MB/s. They are the fastest of the SD card lot and can easily beat a Class 10 memory card which often has a minimum write speed of only 20MB/s. If you come across a memory card that has either the UHS-1 or Class 10 plus the write speed mentioned on it, trust the class marking as the write speed is the potential speed and you may not get it all the time (even with a new card).
The fastest memory cards of all are the UDMA (Ultra Direct Memory Access) CF cards. These can reach a phenomenal speed of 150MB/s and are suitable for professional cameras which come with a CF card slot and of course compatibility with these cards.Remember that CF cards are not the same size as SD cards.