The Na'vi themselves are similar to giant evolved monkeys, similar in many ways to Humans apart from the fact that they have clearly taken a completely different evolutionary track. They have strange tentacles in their heads which are disguised by hair, which they use to interface with the planets network. This odd adaptation is almost unique to the Na'vi and has been widely studied by many scientists, however, due to the hostile nature of the Na'vi few ever get very close.
The Na'vi also have very tough skin and a semi-exoskeleton that protects them from bullets and even energy weapons. However, this does not make them immune. They can be killed by suppressive fire from armour-piercing rounds and sustained direct energy exposure. Despite this, they are still widely avoided. Even races like the Daleks and even the Sangheili. They may be primitive, but their tactics are infalliable.
Na'vi also have a supremely adapted culture, and have a very sophisticated language. Their unity with the planet around them makes them very in tune with the planet and its inhabitants.