What are the differences between various simulator levels?
Here there is a nice tabular representation of the requirements for each level.
Wiki provides a handy summary:

Full Flight Simulators (FFS)

FAA FFS Level A - A motion system is required with at least three degrees of freedom. Airplanes only.

FAA FFS Level B - Requires three axis motion and a higher-fidelity aerodynamic model than does Level A. The lowest level of helicopter flight simulator.

FAA FFS Level C - Requires a motion platform with all six degrees of freedom. Also lower transport delay (latency) over levels A &amp; B. The visual system must have an outside-world horizontal field of view of at least 75 degrees for each pilot.

FAA FFS Level D - The highest level of FFS qualification currently available. Requirements are for Level C with additions. The motion platform must have all six degrees of freedom, and the visual system must have an outside-world horizontal field of view of at least 150 degrees, with a Collimated (distant focus) display. Realistic sounds in the cockpit are required, as well as a number of special motion and visual effects.