Since no one has actually noded anything about the book itself, I think it is only fitting that I should take a stab at it now.
Dune is set in a long gone Past. The past of the storyteller, not ours, as far as WE are concerned it is in the future (around 10,000 years after a huge war known as the Butlerian Jihad which banished all computers and thinking machines).
The known universe is ruled by an Empire, and the main Families constantly jostle for power between each other. The ruling house at the start of the Book is House Corrino, and the head of this house is the Emperor Shaddam. Other houses of note are the noble Atriedes lead by Duke Leto, and the vicious house Harkonnen led by the Baron Vladimir. There are other families who are almost as powerful, and these form a sort of democratic arena to voice their fears and play politics and this is known as the Lanstraad. In the book people who involve themselves in the Lanstraad are generally seen as weak, but I digress.
The other main forces in the Empire are the Bene Gesserit - a sisterhood of mental and physical adepts who have a selective breeding program designed to produce the "Kwisatz Haderach" a supermale who is able to do many spectacular things, not least of which is being able to predict the future in ways the Sisters cannot. There is also the Guild, who hold a monopoly on space travel and guide the space freighters through the inky depths of infinity by their limited abilities at prescience. Running everything commercial is the CHOAM group, a business collective who basically run the economy. The last and major force in the empire is the spice Melange, a geriatric narcotic whose ingestion prolongs life, increases awareness, and allows limited prescience by adepts. Without the spice the Sisters wouldn't be able to carry out their feats of mental and physical agility, the Guild would not be able to carry out interstellar travel, and billions upon billions of people throughout the human universe would die of addictive withdrawal. As you can see, melange is important.
And it is only found on one lonely planet in the whole of the cosmos, Arrakis (or Dune).
It is called Dune for the simple reason that there is no water on its surface anywhere. The whole planet is one large desert. As such water is tightly rationed and sold off, ergo on Dune: water is wealth.
He who controls Dune, controls the spice, and he who controls the spice controls the Universe.
For the past few decades the Harkonnens have been in control of the planet of Dune, when suddenly the Licence to govern the planet and harvest the spice is given to Duke Leto's house. This isn't the victory it appears however, as all sides are aware that the Duke Leto is being set up for a fall, the Harkonnens can legally and are expected to take the planet back by force. After all the two Houses have been fighting for a VERY long time. By placing the Duke in this trap, Emperor Shaddam wishes to remove a House and a leader he sees who may rival him some day.
Anyway, the Duke arrives, prepares for the attack, his son Paul and lady Jessica (Bene Gesserit consort) in tow. Paul is rather special: he has been trained by Jessica, his mother, to be a male Bene Gesserit, with all the skills and mental knowledge that they possess. This is in defiance of her vows to the Sisterhood, but she does it nonetheless. To cut a long story short, (too late, I know) the Harkonnens attack, with the help of the Imperial Commando Troops (known as Sardaukar) in MASSIVE overkill (literally) and utterly destroy the Atreides... well, almost.
Paul and his mother Jessica escape into the desert. They are found by the native Fremen (a cunning and violent people who are used to the ways of the Desert, and who can ride the huge (think Wembley Stadium as mouth size) deadly sandworms that all other people of Dune fear). Their leader Liet Kynes has instructed them to help the unlucky pair, which they do with reluctance. Jessica becomes a priestess for the Fremen, slipping into a role her sisterhood prepared for her. Paul joins into Fremen society and teaches them how to fight and organise themselves. He inevitably becomes their leader, and couples with the dead Liet Kynes daughter Chani. A little while later, he leads them in a battle that destroys the Harkonnen control of Dune, and smashes the Emperor's legions. This (with a little political intrigue and revenge for past grievances) makes Paul the new Emperor of Dune, and since the Fremen know the secret of the Spice, his place is even more secure than the old emperor. End of Story, till the second book.
I am going to stop there. That was meant to be a BRIEF synopsis of the book, but as you can see, to recall even the basic elements of the plot I have had almost too much material. The book is layered, and detailed, and has so much depth that it astounds you from the first page.
There is also this whole air of inevitability about the book which is consistent with its pose as history. It has superb characters, excellent background, is technically amazing, and also introduces some fantastic social and ecological concepts. Not only this, but it manages to reflect on what it means to be human, not in the context of increased technology, for in comparison with many sci-fi books there is very little tech in it, but in the setting of emptiness, and our own inner demons. That there are things we do, that are born of our very natures, and that they fix us on a path that isn't always of our own choosing. But mostly I feel this book is about redemption, not of power or honour, but of the soul. In the way we treat others and ourselves.