Taxonomy is the art of classification - of compiling what Keats described as "the dull catalogue of common things". And meteorologists are experts at it. To the man or woman in the street, for example, any careful distinction between rain and showers might seem a trifle academic, since both, after all, are liable to leave you very wet and render any subtlety of terminology superfluous. But to weather people, the distinction marks a difference of great importance. The difference between showers and rain has nothing to do with the length of time the precipitation lasts. The answer lies in the clouds. Showers, by definition, fall from individual convective clouds, like cumulus or cumulonombus, while rain falls from a layer cloud, like altostratus, spread more or less uniformly over a very large area.
The convective clouds that produce showers have a vertical structure, and tower many thousands of feet into the atmosphere; frequently there is plenty of blue sky to be seen in between them. Rain, on the other hand, is usually formed in the layers of altostratus associated with a front, and which cover the sky like a thick blanket, making the entire landscape dark and threatening. As it happens, shower clouds tend to be individual, local phenomena only a mile or two in diameter, so if the cloud is being carried along by a fresh breeze, the chances are that the shower associated with it will not last long in any particular place, but will have moved on somewhere else in 15 minutes or so. And since rain falls from an extensive area of cloud, the chances are it may take many hours to clear. But on the other hand, if conditions are calm, a shower cloud may sit over the one spot for many hours - and the result is still a shower - not rain. Likewise, rain may fall from one part of a layer of cloud and not from others, so the rain may come and go, but despite its intermittent character, it is still rain rather than a series of showers.
The situation is further complicated when weatherpeople speak of "rain showers". When they do so they are not fudging the fine distinction that they hold so dear, but merely emphasising that such showers consist of drops of water, and not, as they might do in wintertime, of sleet, snow, or even hail. "Small showers last long, but sudden storms are short," declared Shakespeare's John of Gaunt with some authority. But as you can see, the situation is nothing like as simple as the Bard of Avon would like us to believe.