An anachronism is something that is chronologically out of place. For example, a woodstove is out of place in most modern kitchens, just as a microwave oven would be out of place in Renaissance Italy. As writers, we must be careful to keep things where they belong, relative to the calendar, unless we're doing some sort of Jasper Ffordian time-travel or being postmodern. It would not do to have your Regency heroine text her besties about how beastly her beau has been.
But it's not just technology--not just stuff used as set decoration or props--that we have to watch out for. We also have to keep an eye on the images we use in our narratives. We have to make sure that the way we illustrate our story suits the period of the tale.
This weekend, Mighty Reader and I went to a wetland to look at birds. The weather was capricious and at one point as we walked across a field toward a stand of tall trees, the wind came up and the noise of it through the trees was immense and I thought right away of a locomotive. It sounded as if some huge machine was rushing past us and I wondered if I could use that image. Right now I'm revising a novel in which the characters spend a couple of memorable chapters in a wetland/swamp during a period of storms. "This must be," I thought, "Exactly the sound that William heard as the wind blew through the swamp before the hurricane." O, brilliance! O, inspiration!
Too bad I can't use it. My story takes place in 1749 in America, and the first full-service working railway system wasn't built until 1804, in England. Which means that my characters would never have heard a locomotive, and the simile would be meaningless to them. Yes, it would have meaning to my readers, but I believe we writers should show a certain fidelity to our story worlds and that we should restrict our language and images to things which belong to the world of our characters. We need to be careful. To say that Polyphemus threatened Odysseus and his men, raising his great club like a baseball bat, is maybe to use a strong and specific image, but it is not an image that belongs in a story that takes place thousands of years before baseball was invented. It's lazy, authorial sloppiness (though the reverse--a ball player raising his bat like Polyphemus threatening Odysseus with his great cudgel--is fine). So watch it; that's all I'm saying.