Maggie,
Basically, the Preview is the image that will be displayed in the large Viewer pane, while the thumbnail is the smaller image that is displayed in the filmstrip bottom pane. If you have files like JPEGs that have been processed in camera, or imports of other files from things like PS, you can import those to be displayed as is, and not have Aperture generate a new view by reading the entire file and running it through its algorithms.
The Previews that are displayed in the Viewer window can also be generated to specific sizes of resolution, with higher resolution creating larger files and taking longer to generate. When you work on an image file, you are essentially working with the Preview, but the instruction set is applied to the entire image file. The size of the Previews is more related to how you plan to view things while working. So if your viewer pane is relatively small, you do not need to have large previews generated as they will only take up more space and time. However, if your viewer is a second large display, you may want larger Previews generated in order to see the greater details on the larger screen. (When you zoom in to 100%, or use the Loupe, the app is only working on a portion of the file for display, and that overrides your Preview settings while using that mode. As you move around a file that is zoomed in at 100%, Aperture creates/refreshes that portion of the image to fit your Viewer pane.)
Not sure if I am making that completely clear, but that is how I understand how things work. So, if you have the Preferences>Preview set to a large size, the image file stored will be larger, but it still will display to fit the viewer pane. If you set the preference to a smaller size than your Viewer pane, it will not fill the pane. I have been experimenting with using "1/2 size" as the Preview preference. That seems to be an acceptable compromise for speed and detail. There is also the option for overall quality. I have been working at level 10, and that slows things a bit compared to a lower level, sort of like JPEG compression in a way....only an analogy.
So, if you have the horsepower, both in RAM and video card RAM, going to larger sizes at higher quality levels will create outstanding looking images on your display. If you do not have fast processors, lots of RAM and a more powerful video card, such as many laptops have to deal with, dialing things back a bit will speed things up. Aperture uses the CPU, the video card and RAM for its processing more than other apps, so you can tune the app to your set-up and needs. And, lest folks forget, when you plug in a second display, you are sharing the video RAM and processing across both displays. That is another reason why one should select a higher end video card that has more video RAM if they have the option.
O.K., that is about all the Aperture geek I can manage this evening.
LJ