Mike Calligaro wrote a great blog entry on Persitent Store recently and why it's so cool. His latest entry on: RAM, ROM, NAND, NOR is equally excellent. There are many types of memory in mobile devices these days and it's useful to know your acronyms when you go shopping.
I like this simple view of things - "by and large, the amount of RAM you have determines how many things you can run at one time. And the amount of ROM you have determines how much of your stuff you can keep around"