On the PC:
Ever download a cd off the internet and do not have a cd drive or just want to try install software without having to burn off cd and then have to run it from computer. Well WinCdemu will just do this for you, quite tidy on how it works you just double clock on image and it will make the image into a virtual cd drive in My Computer where you can just use it as if you where running it from a computer, http://wincdemu.sysprogs.org/
There are a few other options of software that do the same things like Deamon Tools http://www.daemon-tools.cc/eng/home but this has been coming with Adware lately and is not as double click as WinCdemu.
To copy a full CD onto your computer you can use ISOrecorder, http://isorecorder.alexfeinman.com/isorecorder.htm then once you have the image ISO you can just double click on it and WinCdemu will bring it up in My computer as if it was a CD.
On the MAC:
With Macs images can be mounted and create a CD to ISO by going into Disk Utilties and adding or copying cd onto computer. For more instructions click out http://www.devdaily.com/blog/post/mac-os-x/burn-iso-image-cd-on-mac-os-x/