Sprint used to offer a CF AirCard which we reviewed on this site, but I don't believe they've offered that in at least a year. These days they're all PC Cards which are for notebooks, not PDAs. They have cellular data modems inside and are used to access the Internet (and VPNs and other stuff) using the carrier's wireless connection and service. Sorta like WiFi, only it's available anywhere that carrier has service, rather than being limited to a WiFi HotSpot's 100 foot range.
-------------------- Lisa Gade Editor in Chief, MobileTechReview