There are a lot of tutorials out there on how to burn CDs and DVDs under
Unix-like systems that go into a lot of details about how to do it, but most of
that is really over-kill.  If you're just looking to burn a CD or DVD image 
(iso data file), all you need are the following commands and the name of your 
CD/DVD device:

For CDs: 
cdrecord dev=/dev/??? -data /path/to/cd.iso

For DVDs: 
growisofs -dvd-compat -Z /dev/???=/path/to/dvd.iso

On my current system (Dell Inspirion 1501), /dev/hda is my CD/DVD device, so
using that as an example, the commands would be:

cdrecord dev=/dev/hda -data /path/to/cd.iso 
or 
growisofs -dvd-compat -Z /dev/hda=/path/to/dvd.iso

Failing this of course, you should seek out one of the more complicated
tutorials that describes how to scan for devices, etc.