The SD and SDHC memory card is a highly secure stamp-sized flash memory card, which can be used in a variety of digital products: digital music players, cellular phones, handheld PCs, digital cameras, digital video camcorders, smart phones, car navigation systems and electronic books.To support the higher capacity needs, SD cards are now requiring a different design. 

The new SD 2.0 specifications - which supports 4GB and up - is called Secure Digital High Capacity (SDHC). The SD Association has created and defined 3 speed classes to help you identify speed and performance capabilities/minimum requirements of SDHC cards and their SDHC host products. Many factors impede this performance. SDHC guarantees a minimum read and write speed necessary to handle recording high quality video and audio.