A.E. Van Vogt invented the term "fix-up" to describe a novel made up out of previously published short stories. It was a practice he indulged in himself, quite understandably: for writers who made a meagre living by penning short stories for pulp magazines, it was an efficient and rewarding way of keeping their work out there in hardcover or paperback.
The term is often applied to a number of Bradbury's works, including The Martian Chronicles and Green Shadow, White Whale.
It has been suggested to me that the term is sometimes less than helpful, particularly if it obscures the creative work involved in the "fixing-up" process. Certainly The Martian Chronicles is more than the sum of its parts. The stitching of that particular patchwork quilt of a novel is not utilitarian, but is an important part of the overall weave. To mix a metpahor or two.
Author Michael Swanwick has, I discover, blogged at length about the use of the term - and the abuse of the term. He says it all much better than me, with plenty of examples.