Agile is a technique or a methodology or an approach of developing software, and the Junit is like a framework to program test-cases.
For making an idea about the agile technique everybody talks about today click over to : http://en.wikipedia.org/wiki/Agile_software_development
Junit is usually used by programmers for their unit testing, but I have seen the framework extended so that higher level functional testing can use the same framework. This requires a fair bit of work by the team.
Co-author, with Lisa Crispin: Agile Testing: A Practical Guide for Testers and Agile Teams (Addison-Wesley, 2009) www.janetgregory.ca