I recently was purchasing a high capacity memory card and card reader for a digital camera that uses SD/SDHC. I was a bit confused about the above two card technologies and was not sure if anyone else had the same problem.
In short, SD is limited in maximum capacity and performance due to some kind of limitation. (I think that it is something technical, but am not exactly sure why.) To resolve this. they released SDHC. An SDHC device will read SD cards and SDHC cards while an SD compatible device will only read SD. This same point applies to card readers and cameras.
The general recommendation is that you should stick with SDHC since it offers higher capacities and performance. The issue relates to compatibilty. When making the leap to SDHC, you need to make sure that both your camera AND your card reader support the technology.