To start with, it should be said that Larry Niven invented the "ringworld". For the people who don't know anything about it, I'll explain. First of all, you have to understand a Dysan Sphere. A Dysan Sphere is a huge (I mean really, really big) sphere, with a radius equal to that of earth's orbit. This means, if you can imagine, that all the surface area is a suitable distance from the sun for us to live on. I forget the total surface area, but it is billions of times larger than the earth's populated surface, for sure. The bad thing about this is that you have to have invented gravity generators, and they have to be perfectly flawless. (Another drawback is that you can't see the stars, since you're on the inside). Larry Niven has decided that chopping off the top and bottom hemispheres, leaving only a ring thousands of kilometers wide, would be an ideal compromise. The energy and space gained are still enormous, and for gravity the ring can simply be spun. Make huge walls on the edges to keep in the atmosphere, and you're practically there.
In the story, one of these constructions has been discovered, and a small team of beings is formed to go explore it. The premise is quite good, and the interaction between the characters, who all have a different agenda, is well developed and very interesting. Part of a loosely connected series (of which this book, I believe, is the first), this lays much of the groundwork for some of Larry Niven's later books, (The Ringworld Engineers) which are also quite good.