The slots on the Axim can be used for a variety of things. The main use for most people is for storage memory via SD or CF storage cards. The best way to think of these is as if they are the a hard drive for a Pocket PC. They aren't exacly a hard drive because there are no moving parts, but they serve a similar function. You can store progams, files, etc. on them. For the difference between the two types of cards, read this thread
http://www.aximsite.com/boards/showt...ween+sd+and+cf
As for other uses, on the Axim the CF slot can be used to run other accessories, such as CF Digital Cameras, GPS, Modems, WIFI cards, and many many more options. This is the reason many people prefer to use the SD slot for memory cards. They can leave the CF slot open for accessories.