C＃中的单元测试示例？

What exactly is a unit test and how do I write one? I hear many times people write them before their app is even written, how can this be? I am under the impression that a unit test is some code that makes a call to a method of your app with a set value and expects a specific value to come back, if the specific value does not come back the test has failed. Am I wrong or mislead here? I read so much about unit testing but I know very little about what it actually looks like in code so a sample would be great.What exactly is a unit test and how do I write