Compact Flash (CF) and Secure Digital (SD) are merely form factor standards. This is all you have to know.
Compact Flash = big (compared to SD, anyway) roughly smaller than a GBA cartridge.
Secure Digital = pretty small (too small if you ask me), about the size of a postage stamp.
They both come in diferent sizes like:
32MB, 64MB, 128MB, 256MB, 512MB
I don't know if SD goes beyond that, but CF goes a loong way up to 4.0GB, (anyone feel free to correct me).
Hope this helped.
