A nonconformance is failure to meet specifications or requirements, while (generally) a deviation is permission to proceed (ship product, do further processing, e.g.) when a nonconformance has been discovered.

What constitutes a non conformance?

Non-conformance is a sign that something went wrong in a service, process, product or in the system itself by not meeting a certain set of specifications.

What is the difference between non conformance and non conformity?

regulations, and conforms to its manuals. To put it succinctly, following your procedures is conformity/conformance, not following them is nonconformity/nonconformance. Procedures that meet federal (external) requirements are in compliance, procedures that do not meet said requirements are in noncompliance.