There is no relationship between Selenium and Test-Unit.
Selenium is the overall project which combines RC, WebDriver and Grid.
WebDriver is the newest and more stable piece in the project, allowing you automate a browser using much more stable so-called 'native events'.
TestUnit is simply a unit testing tool for Ruby. You write some tests using Ruby, run it using TestUnit, it verifies if your test passes or fails. You can use this to write Selenium tests.
This is mostly mentioned on the Selenium site: