Navy?What's the difference between cd rom and computers hard disk drive?

I have a New Northstar study guide for e4 exam. And it says . Each time the program is used ,you must use the CD-Rom not the computers hard disk drive.I have an Xp HP tower with 2 drives the bottom sayes Dvd Rom the top sayes Dvd writer. Can i use the dvd rom? it says any attenpt to copy save print anything it will freeze the disk and no refund.