Whilst there are many ways to burn a disk image on a Mac,  here is how to create an ISO image from a Mac using the command line.

Step 1

Insert the DVD or CD in to your drive

Step 2

Open Disk Utility (Click on Launchpad and type in Disk Utility)

Step 3

Click on the CD/DVD drive and make note of the Device.  In this example below it is disk1

Step 4

Open a Terminal Window

Step 5

Type in the following command:

diskutil unmount /dev/disk1 (replace disk1 with whatever your DVD drive is called from Step 3)

Step 6

Type in the following command:

dd if=/dev/disk1 of=DiscImage01.iso

Once finished it will show you something like this:

1637286+0 records in
1637286+0 records out
838290432 bytes transferred in 329.155089 secs (2546795 bytes/sec)

Step 7

Type in the following command:

diskutil eject /dev/disk1

Step 8

Check the directory you were in when you issued the commands.  It will usually be your home directory by default.  You will find an ISO named DiskImage01.iso