The book follows two main protagonists: Anaplian, an agent of the hyper-advanced anarcho-utopian Culture, and her half-brothers Ferbin and Oramen, next in line for the throne of the medieval Sarl.
The Sarl are clients of the crab-like Oct who squabble with another species for control of the Shellworld, an artificial planet created a billion years ago for unknown purposes. The Oct are clients of the insectile Nariscene, and the Nariscene are themselves clients of the spiniform Morthanveld. All this hierarchy is important, because a central theme of Matter is the ways that powerful societies interact with less developed neighbors. Anaplian jumped from the bottom of this hierarchy to the top, and she has to go back to the bottom in order to help her brothers who are the victims of court intrigue.
The gigantic Shellworld is the site of most of the action, and it plays an important role in the story. Ringworld (about an artificial ring around a star) was one of the books that got me hooked on science fiction, so I've always been partial to stories about "giga-architecture". Banks is as good as anybody when it comes to evoking a sense of wonder about impossibly big constructions.
Matter also owes a debt to Fire Upon The Deep by Vernor Vinge (another favorite author). Like FUtD, Matter jumps between medieval and futuristic plot lines; Matter's plot also hinges on the awakening of a formidable ancient artifact. In fact, Banks includes a nod to Vinge when he describes a "Monopathic Hegemonizing Event" -- although Vinge never used that phrase, it's a perfect description of the central threat in his book.
My only complaint about the book is that Banks brings a few important threads to a surprisingly abrupt close. The end comes quickly and I was left wanting more. But overall the book is well worth your time if you have any interest in science fiction.