The Preview window was designed as a fast and fun way to preview animations and get a feel of the physics. It renders in real-time so it doesn't use up RAM memory. It is interactive; either orbit the camera while playing or set the emitter position interactively.
To start preview simply click and hold the mouse button down in the preview window, the cursor will change to a camera orbit symbol and you can freely orbit while the animation is playing. The other way to start a preview is to shift-click in the window, by doing so the animation will play and use the current mouse position as the emitter position. When releasing the mouse button, it will stop emitting and when pressed again it will start emitting. This is a great way to get a feel of how the current settings react to moving the emitter position and starting/stopping the emission.
There are two settings in the preview:
Preview Camera
If there is a camera in the composition you can select to preview the animation as seen from that camera by checking this box. If the comp camera contains animation (it moves or pans) you will see the effect of that motion in the preview. Note that this does not effect the actual rendering, only the preview. In the actual render, the plug-in always uses the comp camera if there is one.
Background Color
Select the background color for the preview by clicking here. This is handy when for example working with black particles, set a light color as background so the particles can be seen clearly. Note that this does not effect actual rendering, only the preview. In the actual render, the plug-in always renders on transparent and creates an alpha channel so the effect can be composited onto anything.