Compact flash (CF) is removable memory used by some navigation systems for storing map data. CF memory modules are about the size of a matchbook and are widely available from stores that sell consumer electronics.
128MB and 256MB compact flash modules work well for storing maps for regional areas. A 2GB module may be large enough to store a full set of maps.
Because CF memory doesn't have any moving parts, it may be better than a hard drive in settings where rough motion and vibration are common (such as an off road vehicle or motorcycle).
Compact flash has lost some market share relative to SD memory over the last few years, but both are still good choices.