The worlds of Larry Niven:
Known Space is the common background for a number of Larry Niven's Sci-Fi stories, which was first begun more than 35 years ago, with new stories added even today. The background includes marvels of engineering, such as the Ringworld, suggestions about human evolution and aging as in Protector, detective stories such as The Long Arm of Gil Hamilton, and a couple of detailed alien races, such as the Kzin, the Outlanders and the Puppeteers. Although there is some unexplaineable technology such as hyperdrives, teleport booths and antigrav, the series is mostly "hard SF", i.e. usually remains within the realm of the explainable, without resorting to technobabble.
History of Known Space:
History begins 1.5 billion years ago, when the Thrints built a galactic empire based on telepathic power, which they used to enslave all other intelligent life, earning them the name Slavers. But when they allowed a certain degree of freedom to the Tnuctipun, a slave race especially skilled in biogenetic engineering, that race used that freedom to free themselves and fight their oppressors. But in the end, they were not able to defeat the Slavers, and suicided all intelligent life in the galaxy.
Humanity has descended from a race known as the Pak, that has three stages of life: infant, breeder and protector. The breeder form is known to us as homo habilis, but to move on into their third stage, they need the help of a virus they call the tree-of-life. This virus was unable to survive on Earth, so the Pak colony failed to bring forth new Protectors. When the old Protectors died, Homo Habilis was left alone and eventually evolved into Homo Sapiens. But in other parts of the galaxy, the warlike pak still exist.
Another interesting twist was that the Puppeteers, a species of cowards, are responsible for giving hyperdrive technology to humanity, in order to tame the warlike Kzin, albeit not on openly. Had they not interferred, the Kzin would surely have enslaved humanity. But they would never admit that if anyone asked, of course. Later, they went on to encourage a special birth control system on Earth, effectively breeding humans to strengthen their foremost talent: Luck.
In general, most of the races in Known Space have compatible DNA, presumably because of yeast left behind on many Slaver colony worlds, which formed the basic building blocks of life as we know it.
Story List of Known Space:
The most popular novel of Known Space is Ringworld, a Hugo and Nebula Awards winning novel, describing an inter-species exploration of the lost secrets of an artifact world. There are rumors about movie plans for Ringworld.
There are also stories from other authors set in this universe, mostly set in the Man-Kzin Wars period, filling in even more detail then Niven alone could.
As for where to best begin reading, I don't know. I started with Ringworld, even if that series is the last, chonologically speaking. But the (unlisted) short story collections Three books of Known Space and Flatlander would probably make good entry points as well, starting earlier. Other than that, you're on your own. But the exploration is half the fun. Happy hunting...