If you want to burn an ISO image to a CD-R/RW or DVD-R/RW, simply open "Disk Utility," drag the .ISO image file into the left-hand sidebar, click on the ISO image in the left-hand sidebar, then click the "Burn" icon at the top of the Disk Utility window.
If you wish to mount an ISO image on your desktop (like any other disk image), simply double-click the ISO image file. You can also perform the above steps, but instead of pressing "Burn," you would press "Mount."
If you wish to create an ISO image from a CD or DVD, insert the CD or DVD, then open "Disk Utility." Highlight the CD or DVD you wish to make an image of, then select "File > New > Disk Image from [CD or DVD name]" where [CD or DVD name] is the name of the CD or DVD you inserted. You would choose "DVD/CD master" for the "Image Format" when prompted. This will create a .CDR image, which is virtually identical to an .ISO.