The old method (of JUnit 3) was to mark the test-classes by extending junit.framework.TestCase. That inherited junit.framework.Assert itself and your test class gained the ability to call the assert methods this way.
Since version 4 of JUnit, the framework uses Annotations for marking tests. So you no longer need to extend TestCase. But that means, the assert methods aren’t available. But you can make a static import of the new Assert class. That’s why all the assert methods in the new class are static methods. So you can import it this way:
import static org.junit.Assert.*;
After this static import, you can use this methods without prefix.
At the redesign they also moved to the new package org.junit that follows better the normal conventions for package naming.