Your axim has a backup battery which contains enough juice to allow the Axim to retain it's data if you have to swap the main memory. So if you have more than one battery you can swap them without wiping anything
If you do a backup and save it on your SD card (with either the built in util or sprite) you should be able to get all of yout info back if it goes completly flat.
BIS is built in storage.. a type of memory that doesn't get wiped when you run out of power.
One of the members here( Stoneraven) has written an excellent memory tutorial for beginners. It helped me out when I just got my axim and you can find a link below
http://www.aximsite.com/articles/link.php?id=253
Hope that helps.