A CPR - Cubase Project is an audio project which was created with the audio production software, Cubase. Cubase is a type of Digital Audio Workstation (DAW), where music or other audio is produced, developed from scratch or enhanced. A file with a .CPR extension typically stores anything pertaining to a specific audio project such as Audio and Midi tracks, mixer settings, any software instruments and audio effects that were used for the project. It also typically contains any references to audio file assets that were stored. .CPR files are those which were created by the fourth Version of Cubase or later, as well as the Cubase Le and SX/SL edditions. It's important to keep in mind that the actual audio file itself is not stored in a .CPR Cubase Project file. Only the afore-mentioned components of the project are compiled. There, if you export a Cubase project to a different location, be sure to move the reference files pertaining to that project as well.
Cubase is a very reliable DAW, and .CPR project files are high quality and easy to develop and use.