A giant cylinder is spotted entering the Solar System and a team of astronauts is sent out to investigate.
The cylinder is unfeasibly vast and (it is discovered) hollow with gravity on the inside of the cylinder produced by centrifugal force. The interior surface is lit by enormous lamps, covered with a variegated landscape and divided in two by a band of sea which exists in a circle around the inside.
Perhaps Clarke's best work, this succeeds (as did Niven's `Ringworld') by its sheer lack of explanation. In fact, the entire novel is, in some ways, an exercise in minimalist adventure, since despite the excitement of the exploration itself and having to rescue a crewmember who becomes stranded on the other side of the central sea, nothing really happens.
One cannot help, however, still being awed by Clarke's depiction of this magnificently vast alien mystery which appears in our Solar System and allows us inside her enormous shell before shortly afterward disappearing.
Again, like Niven's Ringworld, the novel was later lessened by inferior sequels (written in this case in collaboration) and which gradually eroded the awe and mystery which was an integral part of the original books. If you haven't read the Rama sequels you'd be best advised not to bother. The writing is far inferior to Clarke at his best and one suspects that his literary input was minimal.
However, getting back to the original, this is a novel which well deserves the title `classic' and still manages to evoke a sense of wonder set against a background of a universe vast and ultimately unknowable.