When your test plan is ready, you can run it. Test-run is the entity which stores all information necessary to perform, monitor and analyze the process of running tests.
When you choose to run test plan, BTR does the following:
Testers can mark their log entries as passed or failed. A log entry has also an Notes field, which can be used to gather additional remarks about test case.
The tester can assign a bug to test case. A single test case log entry can be assigned many bugs. Of course, a failed test case must have a bug attached. But you can also attach a bug to passed test cases. This makes sense, when your test case itself has passed, but you've detected some bugs as a side effect.
Bugs here, are of course Bugzilla bugs. You have a hyper links from test case log entries to bugs and reverse. You can also enter a bug from the Bugzilla Test Runner test case log entry page.
Your test plan can be run many times. More - you can have many runs of the same plan being run at the same time.
Test-run stores complete information about the status of your test plan at the moment test was run. When you run the test and then change test plan document, or test cases text, this does not affect existing test runs. Test run will keep referring to old values of test plan document and test cases texts.