Sdhc On Sd When Looking At SD Or SDHC Cards They Talk About Class? What Are The Classes And What Do They Mean?

When looking at SD or SDHC cards they talk about class? What are the classes and what do they mean? - sdhc on sd

I have a new Canon Rebel EOS 450D camera SXI. Looking largest and fastest memory card I can get money. And in the online search, I found lots of information, but nothing to compare. The difference between SD and SDHC? Is it only with high capacity? I use the camera? How do I know which card is faster? Is there a difference in quality, too?