Basially, "regular" RAM the kind most of us have in our desktop machines, is too power hungry for PDA applications.
This leaves us with SRAM and Flash RAM. SRAM is faster, can be written to and read from infinite times (effectively), but it tends to be a little more expensive and it requires its own power supply (usually in the form of flat disk batteries).
Flash RAM is a little slower, it has a limited read-write cycle (though this shouldn't be a problem as by the time it gives up the ghost, you'll be running a SuperZoomer 10!), however the thing most people like about it is it runs off the internal power of the PDA and it doesn't need an external battery to maintain it's memory even when not in your machine.
Joe Balsamo (PDC Joe)
PDA Forum Consultant