Handy tutorial from the folks over at the Pi Supply Maker Zone.
You may find yourself needing to backup your SD card for future reference or for posterity and fame. Whatever your reason there are several well documented ways you can do it.
In some case you might also want to be able to get things back from your backup and you then generally need to write it back to and SD card to do so.
Using a Linux distribution of your choice for your desktop system this article shows how to backup a card and access the contents of a .img file. I used Linux Mint but the procedure should be fairly similar for other distributions too.