As always in testing the terms used depend on organisations, however the simple answer is that the test plan is concerned with providing the user will all the information required to perform testing against a specific application / phase etc.
The QA plan could either be for that specific application or phase so would layout the quality expectations from each stage of the process.
That is what I would have said, I would be interested to get other viewpoints on it.
Hmmm. My thoughts on it are a QA Plan is more about how you plan to address the QA needs for the specific cycle. Types of tests you will need to do, when you'll do them, what equipment you may need hardware and software to do the testing, risks involved and plans to mitigate them and time lines and milestones (we will have a 60% pass rate for beta milestone)
A test plan is how you plan to test the business requirement(s) to ensure it is functioning as specified. e.g. Feature A will do B, C, D and E -- My test plan indicates how I am going to validate that Feature A does in fact do B,C,D and E effectively.
A good rule of thumb is to never measure with your thumb.