Standard SD cards can be made with a max capacity of up to 2GB.
SDHC cards come in capacities ranging from “greater than 2GB” to 32GB.
SDXC cards range from “greater than 32GB” to 2TB.

Class 2:  2MB/s minimum write speed
Class 4: 4MB/s minimum write speed
Class 6: 6MB/s minimum write speed
Class 10: 10MB/s minimum write speed
UHS Speed Class 1 (UHS-I): 10MB/s minimum write speed