Nik Define is a piece of software that removes noise from photos. It works pretty well but I don't really notice the differance from what is in Lightroom. However, the great thing about Define is it automatically detects the noise in the photos then automatically adjusts the settings for the noise. I have never really learned how the other features work because I am always pretty happy with the default settings. What it does is find automatically find areas of the photo where there aren't many color changes then detects the noise and auto corrects. It is better than lightroom in this way because you don't have to play with any sliders. For the most part, if I am working with other Nik products like Color Efex Pro then I often end up using Define, but if I am in Lightroom then I just use the Lightroom noise reduction slider.