After Ray Bradbury read Sherman Anderson's Winesburg, Ohio at the age of 24, he was thrilled at the literary possibilities that it opened up for him. Although I haven't read that book, Bradbury implies that what excited him about the book was its beautiful style and fully imagined settings and characters. He almost immediately intended to write a book similar to that one, but with the stories taking place on Mars.
The introduction to The Martian Chronicles goes on to explain that the idea was forgotten after some half-starts and stories quickly tired of. While the idea was consciously forgotten, Bradbury from 1947 to 1949 intermittently worked on a series of what he terms "Shakespearian asides." After a while, these wound up stuck in with a lot of other stuff in a book that Bradbury wound up trying to get published.
The editor, an unrelated fellow named Walter Bradbury, looked the thing over and noticed the preponderance of Martian stories. He suggested that they be bundled separately and that Bradbury write connecting pieces for them, calling it The Martian Chronicles. Ray, remembering the beauty of Winesburg, Ohio that had inspired him to write a Martian book several years before, quite thoroughly agreed. The next day he got $1500 for outlines of both The Martian Chronicles and The Illustrated Man, which he says he lived on for two years.
In terms of plot, there's very little to separate this book from many other sci-fi pieces. Essentially, the humans land on Mars and take it over from the civilizedly lazy Martians, then the humans all move there before destroying themselves. No characters exist across stories, and a few don't even have names, although very few are flat in the English class sense of the term. As one would expect given what he took from his inspiration, the book is very much given to fully fleshed characters, whether the opulently unhappy Martians (who seem to occupy a social niche not unlike recent stereotypical suburbanites, come to think of it) or the later human occupants of his Mars.
The real beauty of this book, though, is the incredible descriptive style of Bradbury's writing. The prose is very lively and incredibly dense in meaning. Periodically, just when you're starting to get used to the immersive language, Bradbury will throw in something so pretty that it'll damn near make you cry.
On a totally subjective note, I should say that this is one of my favorite books of all time. I acquired it from a girlfriend who gave it to me for Christmas. That girlfriend broke up with me a week later on New Year's Day after having spent the night before making out with the 31-year-old brother of one of my friends (she was 15 at the time). I believe it speaks to the quality of this book that I, as an incredibly angry 16-year-old, did not as a result do anything rash to the copy she had given me. I had already read the thing in the intervening week and I quickly decided that she wasn't worth losing it for.