It is sometimes described as a donut shape but a donut shape is only the simplest of the toroidal shapes. The shapes pictured in this blog also define toroids.
The best calculations by mathematical physicists tell us that the universe is close to completely flat and could fit on the surface of such a toroid. This is a shape that is definitely hard to conceptualize for very long. I think it must have been brief glimpses into the mysteries of the universe such as this that led Albert Camus to say, “Beauty is unbearable, drives us to despair, offering us for a minute the glimpse of an eternity that we should like to stretch out over the whole of time.” We can only hold such images in our minds for the briefest of moments before they escape us or drive us mad. Those who lived in a "flat disk world-view" must have felt the same. How could one hold onto this concept for more than a few seconds? It was outside of the experience of all people until a few brought back reports of strange journeys and new understandings of geography and geometry. Now, although we take it for granted that it can be done, few have circumnavigated the globe to prove the theory. I can only dream of what the experience might be like and only astronauts have a true, immediate, experience of circumnavigation. But, in my mind, momentarily, I can capture the mystery.
Nature; Published online 23 May 2008 | Nature | doi:10.1038/news.2008.854; "Doughnut-Shaped Universe bites back: Astronomers say Universe is small and finite."