Your master files contain all the work you did in camera. They are your image files produced by the camera.
Your library contains all the work you did on the computer. Things like rating, edits, slideshows, albums, etc.
If you export the master files, you are only backing up the master files, not all the work you did on the computer.
To make a backup copy of your library, don't use export, use finder and make a copy from there (export works as well, but gives a new internal library ID so is not technically an exact copy). By making a copy, this backs up all the work you've done on your computer and probably your master files as well (see next comment).
You also need to understand the difference between Managed Files and Referenced Files. You decide this when you originally import the files into Aperture. By default, it will copy the images into the Aperture library (managed files), so when you copy the library, you are making a copy of both your images and all the work you did on the computer).
However, if, when importing, you chose not to store the image files in the library (Referenced files) then making a copy of the library only backs up the work you did on the computer, not your master files. So you'll need a backup copy of those too, either by copying the folder that contains them in finder, or by using Export Masters, or using your backup software.
Andy
