In the Mahabharata the Naga Kingdom is the territory of a hardy and warlike tribe called Nagas. They were also considered as one of the supernatural races like the Kinnaras.
The Sanskrit word Naga
The word Naga in the Sanskrit language means snake or serpent. It seems likely that the Naga people were a serpent-worshipping group who were later described as serpents themselves in ancient Indian literature. Ayus people were Serpent cult people from the beginning. Serpents are reptiles with "hoods". Snakes are reptiles without hoods. Maha Vishnu's Shesha NAAG was a huge serpent with 5 hoods. That needs to be differentiated from Shiva's one headed snake.
References
Naga Kingdom Wikipedia(Text) CC BY-SA