From the commercial print industry here comes another brilliantly confusing bit of terminology to understand. Namely variable printing.  As can be found in the dictionary, variable means “capable of being  changed”, so the basic meaning for the phrase becomes “print that is  capable of being changed”. 
Let’s walk through a real-world  example where variable printing would be used: 
You want to mail out thank you postcards to a group of clients that  hadn’t ordered from your company in quite some time. Can you imagine how  much time it would take to write all of those postcards and then  address them all? One way to cut down on a lot of the work would  be to create just one message and pre-print it on all the postcards, and  then all you need to do is manually add all of the recipient’s  names/addresses by hand. Hang on though, you could also use your PC to  output all of the recipient’s names and addresses on adhesive label  stock and then stick the labels to the postcards? Would that not save  even more time and effort?
Unfortunately, the down-side is the  fact that your postcards are going to look to be mass-produced with very  little thought about providing a personal touch so the chance is less  that your marketing effort will make some sort of meaningful impact on  the recipient.
Now, what would be the solution? If you thought it might be variable printing you’d be correct. 
Variable Printing is an extension of digital print technology that joins a computer  spreadsheet file with a digital print press via specialized software.  The information held within the spreadsheet is included automatically  into the design without any interruption of the printing press. This  then allows each and every printed piece to be completelyunique no  matter the number ‘up’ on a sheet, or the number of sheets in total.
So,  in the case of your thank you postcard, you would have to provide the  artwork for the postcard in addition to a separate spreadsheet file (for  example: xls, csv, etc.) that holds all of the variable data. The most  often used info for variable printing is the recipient name and address,  but you can also customize a number of elements within the message  which happens to include both the front and back covers.
As an  example, imagine the impression that could be made when someone gets a  postcard addressed to them (and let’s just say that his name is… John)  with the cover titled, “John, we’d like to thank you for your business!”  The client will have the perception that you cared sufficiently about  his business that you wanted to contact him by his name, call him by his  name, as well as thank him by name.