There are two areas in LR when you develop your image where you can adjust these parameters, one in the curves section and one overall I think. What's the difference and when should you use which?
The two main area sliders have preset brightness points they act on, they are the 'quick and easy' solution.
The curves section gives a lot more control, as you can position not only the sliders, but also their attack points (in the diagram, click the little triangles at the bottom and drag them left/right).
If you move them exactly to the predefined settings of the main area sliders, there is no difference in their activities.