The White Walkers are important. Though they've only really started cropping up more frequently now that we've reached Season 5, they've been a part of the show since the first scene of the Pilot episode and have been an ever-looming presence since. Their significance will undoubtedly continue to grow in the sixth season. How did Martin come up with such terrifying creatures? It seems that he drew inspiration from the aos si, a supernatural race that shows up in Irish and Scottish mythology. The aos si are like fairies or elves, and are said to live in an invisible world that coexists with the world of humans. What are the similarities? Well, they're often appeased with offerings, just like the White Walkers accept child sacrifices. Not only that, but the Irish mythological invasion cycles parallel the invasions of Westeros Ireland was repeatedly invaded by groups of fairies (also considered gods) that were often described as monstrous, which ultimately resulted in a single group seizing control of the island. This is similar to the Children Of The Forest, and the invasion of the First Men.
