The Celts were a group of peoples loosely tied by similar language, religion, and cultural expression. They were not centrally governed by one all powerful ruler, but smaller clans and tribes governed themselves. They are thought of as warriors, living for the glories of battle and plunder, but as much as this may be true they also loved art/literature and held deep religious believes. They were also the people who brought iron working to the British Isles.
Celtic lands were owned communally, and wealth seems to have been based largely on the size of cattle herd owned. The lot of women was a good deal better than in most societies of that time. They were technically equal to men, owned property, and could choose their own husbands. They could also be war leaders, as Boadicea later proved.
The Celts did have a written language, but it developed well into Christian times, so for much of Celtic history they relied on oral transmission of culture, primarily through the efforts of bards and poets. These arts were tremendously important to the Celts, and much of what we know of their traditions comes to us today through the old tales and poems that were handed down for generations before eventually being written down.
Another area where oral traditions were important was in the training of Druids. There has been a lot of nonsense written about Druids, but they were a curious lot; a sort priest, political advisors, teachers, healers, and arbitrators. They had their own universities, where traditional knowledge was passed on by verbal means rather than written. They had the right to speak ahead of the king in council, and may have held more authority than the king. They acted as ambassadors in time of war, they composed verse and upheld the law. They were a sort of glue holding together Celtic culture. From what we know of the Celts from Roman commentators, who are, remember, witnesses with an axe to grind, they held many of their religious ceremonies in woodland groves and near sacred water, such as wells and springs. The Romans speak of human sacrifice as being a part of Celtic religion. One thing we do know, the Celts revered human heads. Celtic warriors would cut off the heads of their enemies in battle and display them as trophies. They mounted heads in doorposts and hung them from their belts. This might seem barbaric to us, but to the Celt the seat of spiritual power was the head, so by taking the head of a vanquished foe they were appropriating that power for themselves and would stop the spirit of the defeated enemy working against them in the other world.
The Celts loved war. If one wasn't happening they'd be sure to start one. They were scrappers from the word go. They arrayed themselves as fiercely as possible, sometimes charging into battle fully naked, dyed blue from head to toe, and screaming like banshees to terrify their enemies, both men and woman alike.
They took tremendous pride in their appearance in battle, if we can judge by the elaborately embellished weapons and paraphernalia they used. Golden shields and breastplates shared pride of place with ornamented helmets and trumpets.
The Celts were great users of light chariots in warfare. From this chariot, drawn by two horses, they would throw spears at an enemy before dismounting to have a go with heavy slashing swords.