To prove that the instrument concept works and can provide the intended measurements.  The breadboard will probably not look like the finished instrument, and will use different components, but will have the same basic operation.  The breadboard is used throughout development program as it forms the testbed for FPGA software development right up until the Qualification model (and usually the Flight model) parts are burned.

This model will have representative mechanical and thermal properties to the final instrument. It is integrated onto a dedicated spacecraft structural and thermal model to allow complete testing of the spacecraft mechanical and thermal behaviour, but will not usually be able to make any measurements itself. 

This in one stage on from the breadboard, and will have the same functionality as the finished instrument.  The electrical model will demonstrate that the instrument design can be integrated onto the Spacecraft data and power buses.  The parts used here will be commercial versions of the space qualified components intended for the final design.

This model will be fully representative of the final instrument.  The components used, will be of the same type and specification as the models built to fly on the spacecraft, and will be parts specially designed to survive in the harsh space environment.  This model will be subjected to testing up to the limits of the operational and non-operational environmental conditions specified for the instrument to endure.  This will include thermal cycling in a vacuum and vibration testing.  The Qualification model is also subject to Electromagnetic compatibility (EMC) testing to ensure that the radio frequency emission from the instrument and the susceptibility of the instrument to incoming electromagnetic energy are below specified levels.

This is the instrument model that will fly on the spacecraft.  It will be of the same design, and using the same specification components as the Qualification Model.  It will be rigorously tested up to Acceptance Level environmental conditions; Acceptance levels are wider than expected operating conditions, but not as extreme as Qualification levels.

This instrument will be representative of the flight instrument in every way and is used for any testing that must be done on the ground to prepare for in-flight operations once the mission has launched.