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The technology is such that RAM, the volatile kind that goes away when power fails, is faster and consumes less power to read/write. The downside is that it has to be refreshed frequently to keep it working. I think in the normal operation of the Axim it's refreshed something on the order of once every 8 clock cycles. If that refreshment does not occur, the charge starts to die and data is lost.
The NON-volatile memory, ROM, CF/SD cards, etc. is much slower to read and write and consumes more power to do that read and write. The upside is that once written, it survives power failure and doesn't need to be refreshed.
So, basically, even when you "save" to the main memory, it's in a volatile media unless you do something to move it to non-volatile memory. That "something" is called backup.
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