One of the often overlooked features of memory cards is the Class system. Usually we think of size and compatibility when choosing a memory card for our digital devices, but class is just as important. There are currently five classes; Class 2, Class 4, Class 6, Class 8 and Class 10. This class rating is actually a speed class rating, and represents the amount MBs transferred per second. So, Class 4 memory cards, such as Samsungs SD cards, transfer at 4MBps and Class 10 memory cards, which are the latest on the market and ideal for devices such as DSLR cameras, transfer up to 10MBps.

Five classes of memory cards in the market Class 2, Class 4, Class 6, Class 8 and Class 10
so rest you can understand yourself sir

Memory Cards other than the storing capacity are also defined by the data transfer speed by class 4, class 10 etc. Higher the class, higher is speed. It is measured by how quickly information can be read from, or written to.