There is a lot of discussion about this blog - http://www.artima.com/weblogs/viewpost.jsp?thread=126923

Which talk about classification of functional test and unit test. Nice to read. You can also refer to the following article:

http://beust.com/weblog/archives/000319.html
http://www.magpiebrain.com/archives/2005/09/18/unit_tests

Checking vs. testing - http://www.developsense.com/2009/08/testing-vs-checking.html

Focusing on testing - http://fabiopereira.me/blog/2012/03/18/introducing-depth-of-test-dot/

Verification and Validation in Testing - https://www.pcloudy.com/blogs/verification-and-validation-in-testing/