SDHC is SD flash memory card?

You might have heard about Secure Digital High Capacity (SDHC) flash memory, do you know the differences between this SDHC and Secure Digital(SD) card?
Of course they do have big differences between them.
A card reader might not be able to read SDHC while work well for SD card, but it's already a past, many devices have become SDHC compatible nowadays. However, older SD compatible devices(example: digital camera) don't normally support SDHC card, so check the devices' memory card compatibility first before purchasing SDHC card.
The picture, show a SDHC card with 4GB capacity and the 4 within a circle implying it's class speed rating.
High capacity of SDHC is in the range of 4GB to 64GB or maybe higher in the future.
What is Class Speed Rating?
This is what developer can choose which data-writing speed specify to a card, so a garanteed minimum data transfer rate is fixed.
There are 3 classes, which is Class 2, Class 4 and Class 6.

For more detail, you can download a pdf file from here.