A DVD±RW drive can burn CDs and DVDs, while a DVD-ROM drive can only read CDs and DVDs. What model iMac do you have and what OS version is it running? The available bus options (ports) on your iMac, will determine the type of burner you should use. If you have FireWire, you'll be better off than having only USB, which on older iMacs is the slower USB 1.1. A USB 1.1-connected CD-R/RW drive is limited to burning at 4x - which is tolerable, but I wouldn't even try to connect a DVD±RW drive to burn a DVD, because of the larger amount of data to be burned and the speed limitation. I would expect data overrun problems, resulting in too many coasters. FireWire is a much faster bus than USB 1.1, and it can easily handle burning CDs and DVDs, but the OS version is also a factor for drive support. If you post back with details, it will fine-tune the suggestions made.