A good article explaining difference between RAM & ROM http://blogs.msdn.com/windowsmobile/archive/2005/08/19/453784.aspx

Main differences are (taken from article)
RAM: very fast, but burns a lot of power.
ROM: much slower, but burns very little power.
Importantly, RAM needs a constant supply of power for it to remember its data. ROM doesn't. In other words, if your batteries die, you'll lose the stuff in RAM, but the stuff in ROM will still be there.