Rob did an excellent job of describing what sampling is. I'll add a couple other comments though.
Depending on how severe you are undersampling the majority of the star's energy distribution (called the point spread function or PSF) may fit inside a single pixel. This will give you 'square stars' for a lot of the fainter stars. Usually wide field exposures have this effect but in general the small stars are lost in the greater field of view so it does not detract from the image. Undersampling can be improved some by techniques like dithering/drizzling but at the cost of making the signal noisier as well requiring significantly more memory and disk space for the image data.
Oversampling isn't necessarily a problem either. You will end up with images where stars cover several pixels. Some prefer the appearance of images like this. It can also make processing an image a bit easier.
The entire system is important not just the focal ratio of a scope. Ultimately the environmental conditions (seeing, tracking, etc.), aperture, focal length, pixel size and camera characteristics all combined determine how quickly you will achieve a given signal to noise ratio (SNR - a measure of how noisy your image will look), how well resolved the image will be and what your field of view is.
There's a lot of math behind this sort of analysis but it isn't absolutely necessary to understand it all, although I encourage people to try. What is really important is what equipment you already have and what objects you are interested in capturing. Now if your goal is to capture the smallest structures possible for your given area then I would tend towards over sampling. If you want to capture wide field objects, don't worry about the sampling, just target the field of view that captures what you want.
EDIT: Rick makes some excellent points as well. His comment about sampling at 1/3rd the seeing is derived from the fact that we are not sampling a one dimensional structure. We have to take the diagonal inbetween pixels into account. Since we are typically using square pixels the ratio of the diagonal to one of the square edges is SQRT(2) or 1.414... Since sampling theory requires twice the sampling of the highest frequency element this really becomes 2.828 times or very close to 1/3rd of the seeing (or diffraction of your optics whichever is the limiter).
Edited by David Ault, 19 March 2016 - 12:48 PM.