What does the Render and Preview of the Samples section do? Yesterday, i put my animation to render, it went fine. But one thing that bothers me is: What is the Render and Preview? I actually discovered that the Render is something like the quality, so i keep it on 128 because it is fast and the quality is great for me. But what is Preview? And what should i keep it on? Any help would be appreciated.

I use the CUDA with GPU Compute rendering (cycles). I'm new to Blender so i may seem silly.
The Render samples is for your final render when you hit the render button

The Preview is when you switch your 3D view to be rendered. Its a quick view.