Screening and diagnostic tests, from Medscape: If you want to dig more into the differences between screening and diagnostic tests, this overview on Medscape reviews not only those differences but also the difference between specificity and sensitivity and how to understand positive and negative predictive values.
Making sense of screening: Understanding the difference between screening and diagnostic tests and the risks and benefits of screening tests is essential to reporting on them accurately. This page contains a link to a complete guide on screening tests from Sense About Science as well as the basics nicely presented in this slideshow (PDF)