A test plan is a predetermined list of testing steps that need to be performed before shipping a product to the general public. For example, a company may create a test plan for a new hardware product they are developing. The test plan may contain what needs to be tested and how it should be tested. An individual, or group of individuals, then goes through that list verifying that each of the tests works properly with the new hardware.
Test plans help a company identify what needs to be tested, what has already been tested, how a test was performed, and the progress of the overall testing before shipping a product. Unlike a standard test, test plans are often more in depth and contain several hundred or thousands of different items that need to be tested.