The story is (very) loosely based on England's War of the Roses. In the 15th century, it was the House of York versus the House of Lancaster. In Game of Thrones, it's Stark versus Lannister.
To that, add something like Hadrian's Wall, which, in real life, kept the Picts at bay. Martin's "Wall" keeps the "wildlings" and "the Others" at bay. Toss in the Mongols--the "dothraki"--as well as dragons, seasons that last decades, and the machinations of medieval politics, and you have begun to enter the world imagined by George R.R. Martin.
It's a dismal one. The violence is frequent, masochistic, and described with relish. It gets a bit much, frankly, but that seems to be one of Martin's main points. The ethical man--Lord Stark--is beheaded right off the bat, on the whim of a testosterone-addled teenager. Life is "nasty, brutish, and short," as Thomas Hobbes once put it, and Game of Thrones underlines the point repeatedly.
Through five volumes, the poor Starks have endured much. Lord Stark, the Hand of King Robert, was beheaded. His son Robert, the King of the North, was killed at the Red Wedding, as was his mother, Catelyn. Bran was thrown out of a window at age 4 and crippled for life. Rickon is presumably dead. Arya was last seen mind-merging with a tree.
Jon Snow, Lord Stark's bastard son, is apparently assassinated by his colleagues at the end of volume 5. I say "apparently" because Brienne of Tarth was presumed dead as well, but managed to reappear somehow. Snow was last seen being stabbed by several daggers, like Julius Caesar, but was not actually pronounced dead. Maybe Jon Snow will reappear in upcoming volumes 6 and 7.
Volumes 1-3 were compelling. The main characters are interesting and well-described. (Lesser characters number at least in the dozens, if not hundreds.) The narrative moves with energy along multiple trajectories.
Everything seemed to fall apart after the Red Wedding. The assassination of young King Robert was a shock, especially since it seemed King Robert was the one perhaps most likely to win the "game of thrones," the battle for sole kingship in the seven kingdoms.
After the Red Wedding, the narrative loses its forward movement. The narrative bogs down and nobody really seems to do anything. They move around a lot. Various characters are on various difficult journeys of great length, described often and in great detail, in which every moment is fraught with sudden danger and arbitrary violence.
Yet, nothing actually seems to happen. At the beginning of volume 5, Lord Stannis was in the snows outside Winterfell, precisely where he would be at the end of volume 5.
Volume 6 is not due for a few years, to be followed by a final volume 7. Perhaps that will give the author sufficient space to bring at least a few of the many threads together. Perhaps he will hew to his line of life's dismal suffering and drudgery, or perhaps Arya Stark will reclaim Winterfell!
Volumes 1-3 were riveting. Volumes 4 and 5 were rather a slog. Here's hoping the final volumes are a return to the series' former glory.