I know little about the field, so in my mind that qualifies me to have an opinion.....
I think there are many CAD programs, each trying to increase market share. Each of them use a proprietry file format (though there are open source CAD programs as well). In order to get market share, each CAD program will usually read several competitor's formats, and output in a few other formats apart from their proprietry one. The most comon 'standard' format is .DFX, which most programs will read, and which is often used as an 'interchange standard'. CAD programs will usually output as JPG or PDF as well.
A CAD format is usually a 'better' drawing - you can do more with it, but only if you've got the right CAD software. And CAD software, even the easiest, can be a little hard to learn how to use. If you just need a standard print, .PDF is fine and you can ignore the .CAD. If you want to alter the drawing or play with it, you will need to use the .CAD software.
You can get an idea of what CAD program was used to create it by looking here:
http://filext.com/file-extension/CAD . It might be QuickCAD or CADstd.... You can often download free readers, restricted function 'lite' versions of CAD packages, or trialware and read the file that way. But it will probably take you a while to work out how to manipulate the image....