Integration tests are those that integrate with an external system. For instance, a test that interacts with a database would be considered an integration test. The external system doesn't have to be a third-party product; however, sometimes, the external system is just an imported library that was developed independently from the application you are working on but is still considered in-house software. Another example that most don't consider is interactions with the system or language framework. You could consider any test that uses the functions of C#'s DateTime object to be an integration test.