Both HOWTOs assume the user knows Java, but not JUnit.
Available at http://pub.admc.com/howtos/junit .
Besides explaining basic usage, the 3.x Howto explains the tricky aspects of 3.x, such as organizing suites and making test classes testrunner-friendly.
Besides explaining basic usage, the 4.x Howto covers good test design and the distinction between JUnit failures and errors.