dd if=INPUT-FILE-NAME of=OUTPUT-FILE-NAME

To make an ISO from your CD/DVD, place the media in your drive but do not mount it. If it automounts, unmount it.

dd if=/dev/dvd of=dvd.iso   
 # for dvd dd if=/dev/cdrom of=cd.iso  
 # for cdrom dd if=/dev/scd0 of=cd.iso   
 # if cdrom is scsi 

To make an ISO from files on your hard drive, create a directory which holds the files you want. Then use the mkisofs command.

mkisofs -o /tmp/cd.iso /tmp/directory/

This results in a file called cd.iso in folder /tmp which contains all the files and directories in /tmp/directory/.