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With a perfect optics and detector stars should look like Airy disks. If you use a telescope other than a refractor, they should look like the equivalent of an Airy disk for your scope (in more precise technical terms: the square magnitude of the Fourier transform of the aperture function of your scope).
But with practical detectors (CCDs etc) the signal will bleed to neighbouring pixels. That's both good and bad: good because in the image brighter stars will look brighter; bad because the extreme brightness of stars is only captured by bloating their imprint on the pixel array. It's a dynamic range issue and can to some degree be tamed with HDR techniques.
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