NCover is a tool that analyzes running code and is able to determine whether code was visited or unvisited while running. Visited code is code that was executed during the running of a process, and unvisited code is code that was not executed.
Think about it for a second, and think about it in terms of running automated tests: If you can run automated tests and see what code is visited and what code is not visited, you can make use of this information and make sure that all code is visited--or at least a good portion of the code, depending on how far you want to take your automated tests.
NCover suggests you cover 95% or more of your code, but I'm not here to tell you how much or how little of your code should be covered; I'm here to help you get up and running with NCover...the rest is up to you.
You'll see some links to the left under External Links that take you to places where you can download NCover and NCoverExplorer. NCoverExplorer is a front-end that displays the XML output of NCover in an easy to read form. NCover is a command-line driven tool that outputs its result in XML, so reading it and making use of it is difficult at best. By utilizing NCoverExplorer, which has a simple but very useful GUI, you'll make the most of NCover and you'll take your automated tests to new heights.
At the very moment TDD fully clicked in my head, I remember thinking, "Whatever it was that I was doing before TDD surely wasn't real software development," I had a similar thought when getting NCover and NCoverExplorer running on a real-life project: "Whatever it was that I was doing with TDD surely wasn't real TDD." The results of NCover were very revealing to me, and it has taken my TDD skills to new levels.