Rama is a large cylindrical spacecraft designed for the transportation of large numbers of alien life forms. It uses rotation to generate gravity and has a reactionless drive system.

On it's first visit to our solar system, a group of humans visited it, explored it and went home before it dove into the sun to refuel then left. Humanity went crazy and entered into a massive depression.

The ship came back many years later. A group of humans visited it, had sex, killed a few of their number off, tried to blow up the ship, some of them went back to Earth, three remained on board and managed to warn it of the oncoming nukes. Much sex and strange aliens (not together mind you) ensued.

The ship came back AGAIN, picked up a few hundred humans bound for Mars. MUCH more sex, murder and drug use ensued. Humans who had managed to Behave Yourself were given the choice of going back to Earth or going to a big space station, all the others were sent back.

I strongly recomend the story of the first visit Rendezvous with Rama the later books, and a sequel which doesn't directly involve the Rama spacecraft, had too much sex and violence, and not enough of the cool aliens.