And there is another card type, the "CF" or "Compact Flash" card, which can be any number of things, including additional storage, wireless communication, modem, etc. You can read more about CF at
http://www.compactflash.org/faqs/faq...haracteristics
For the AXIM, the "SD" slot can accept only "SD" cards -- which means you will want to use the SD slot for additional storage. The "CF" slot can be used for any number of things, depending on your use of the AXIM PocketPC. You could use CF for additional storage, but many opt to use it for other functionality.
I think a big selling point for the AXIM is the inclusion of both the SD and the CF slot at a reasonable price point - it provides the end user with quite a bit of flexibility and portability. (Other PDA-type devices require ad-ons to take advantage of both card types, or if they have both, said devices are much more expensive.) It's also why I purchased the AXIM - absent this functionality, I'd probably be purchasing an ultrathin laptop.