None, other than the title of the checklist. They are both a way to inform the field inspector what items that the purchaser considers significant, and how they want them checked.

Not sure what part of the world you are from but in the Southern Hemisphere there is quite a bit of difference (if I am interpreting QAP correctly)
The Quality Assurance Plan (we call it QMP - Quality Management Plan) basically is the overriding document that describes how you will manage the Quality System.
As example
Project Management Strategy
Applicable documents
Management Systems
Management Responsibilities
Resource Management
Product Realization
Measurement analysis and improvement

whereas the ITP (Inspection and Test Plan) is a document that describes the various stages of inspection and testing from start to finish of the manufacture of an item.
Generally this will include Monitoring, Review, Witness and Hold Points listed for all relevant parties and this is approved prior to commencement.
Hope that helps,
Cheers,
DD