I have heard some people saying that Unit testing is done primarily by the developers and test engineers need not know about Unit testing. But this is not the case, Unit testing is as important for test engineers as it is for developers.
Probably developers will write the unit test cases but your understanding of the framework and unit testing can certainly help him/her in designing the flow for unit test cases, generating test data and making good reports. All these activities will ultimately help you in the long run as product quality will increase significantly because of the efforts you put in on the unit testing. So if you think as a test engineer, you should also learn/know about unit testing read on.
Unit testing is the method of making sure that smallest unit of your software is working properly in isolation. You can define this smallest unit in your own term, it could be a java class or library. A more formal definition and one of the earliest paper on Unit Testing by Kent Beck can be found here.
Kent Beck wrote JUnit framework along with Erich Gamma and it became extremely useful for the Java community. JUnit made it very easy for the developers to write, organize and execute unit test cases for there java code. Success of JUnit was enough to inspire people working on other languages to come up with unit testing framework for the language of their choice. Now we probably have more than 25 Unit testing frameworks based on the XUnit architecture covering all the major languages. Information on unit test frameworks available are present here.
Information about JUnit and Jtescase is also present here, because these are the most widely used unit testing frameworks. If you want me to add more information about any particular unit testing framework, contact us.