Our information about the druids are pretty scarce because the druids themselves didn't keep any records. What knowledge we have are from the Romans, who usually portray them as some sort of priests (divine powers, contact with the Gods, etc.). This quote is from Tacitus (book XIV, chapter 30) in the Annals:

On the shore stood the opposing army with its dense array of armed > warriors, while between the ranks dashed women, in black attire like > the Furies, with hair dishevelled, waving brands. All around, the Druids, lifting up their hands to heaven, and pouring forth dreadful imprecations, scared our soldiers by the unfamiliar sight, so that, as > if their limbs were paralysed, they stood motionless, and exposed to > wounds. Then urged by their general's appeals and mutual encouragements not to quail before a troop of frenzied women, they > bore the standards onwards, smote down all resistance, and wrapped the foe in the flames of his own brands. A force was next set over the conquered, and their groves, devoted to inhuman superstitions, were destroyed. They deemed it indeed a duty to cover their altars with the blood of captives and to consult their deities through human entrails.

As you can see, the druids were thought to have some sort of power that made the enemies weak and after their defeat the Romans simply burned the druids' grooves (sort of their 'temples'), which may be why we don't have any information about them. Perhaps I should clarify about the quote as well. The last few lines about covering the alters in blood and consult their deities through human entrails are describing the Romans, not the druids.

And if someone can explain to me how to format a quote that long without getting random arrows in the middle of the lines (which it seems I need to make it a quote), I would be most grateful.