An interesting bit to keep in mind, of course, is that D&D druids and bards have only the name in common with historical druids and bards.
Another is that modern druids are, like D&D ones, totally unrelated to historical druids.
We don't know much about ancient druids. They were the councilors of branes, celtic king. They weren't tree-hugging hippies, they weren't shapechanging animal-buddy.
They performed a number of rituals, notably the sacrifice of prisonners and criminals.
Presumably, they were supposed to foretell future.
Also, they were, like voodoo priests much later, experts in drugs. Rather than Asterix-style invulnerability potions, they created hallucinatory drugs that induced frenzy. It was great in combat: imagine a horde of screaming, naked barbarians, charging toward your disciplined, civilized squadron. The fastest of the barbarians, insanity gleaming in his eyes, seize your pilum in his barehands, and those of your comrade, and with mad laughter, thrust them all in his own belly, before trying to hug you.
Your refined roman mind snap. Those are not humans, those are not beasts, those things you have to fight are insane monsters.
And now that you're disarmed, the rest of the bunch is on you, swinging their swords and axes.
Of course, once the inital shock has passed out, and the romans became accustomed with these peculiar fighting method, the efficiency of the whole scheme lessened. Gradually, the gauls had to realize that against an opponent that is not as easily demoralized as before, armour and tactics were better than divine blessing and drugged bravado.
What else? Druids believed in the power of words. Words are the most powerful magic of all. The power of words is great, and the power of written words is even greater. One should never write anything pertaining to the gods, to magic. Albeit the druids knew of writings, their religion forbade them to ever rely on anything else than oral transmission of lore.
That's why we know next to nothing of them.