For a concise answer it would be helpful to understand the context of your question: what do you mean, "What is a Druid?" Lacking that we'll have to frame our answers in several of the possible contexts one might encounter....

1. In the Ancient World:

The Druids were the priestly caste of the Celts. The Druids served as the people's religious leaders, judges, doctors historians and scientists. A highly respected subclass of their culture/the intelligentsia functioning in service of their people and the gods. The best place to read a contemporary description of them is in Julius Caesar's The Gallic Wars. The Celtic peoples* were the primary inhabitants of Europe from 1200 BC through 200 AD (stretching through the area north of Italy and Greece yet south of Scandinavia/and from just west of central Hungary -all the way west through Austria, Germany , France and Spain and throughout the whole of the British Isles, arriving in Ireland last). Except for Ireland, Scotland and Wales, the prevailing Celtic cultures eventually were assimilated into the Roman Empire and diffused during a transitional phase called Romano-Celtic from roughly 100 BC through 200 AD. We can assume the original Druids had disappeared by the end of that period. In the far west, particularly in Ireland, they may have survived longer, finally converting to Christianity in the early Dark Ages.

*a linguistically linked set of cultures (not a "race" or bloodline...)

2. The Romantic Tradition:

In England, France and to a lesser degree Ireland in the late 1700's there started a resurgence of interest in this ancestral heritage. The Druids were seen as the holders of the Ancient Mysteries of Europe and the British Isles and were (incorrectly) associated with the people who built Stonehenge and the other prehistoric monuments of their countryside. "Druid" fraternities sprung up in London, Paris and other cities around the beginning of the 1800's and their ceremonies were very much a blend of Masonic lodge rite stylings, mystical Christianity, bad scholarship and bad poetry. It wouldn't be correct to refer to these groups as "paganism". Lodges which are the descendants of these groups (one or two) still survive in Britain and France today.

3. Modern "Druidism":

Neopagan groups using the term Druidism to describe their activity have been springing up in America since the early 1970's - and the effect has been rolling back to Britain and the Continent. The stodgy old Christian mysticism and Masonic heritage is being revised into a post-60's non-christian nature religion which since the 80's has itself been going through a serious revision towards a more scholarly-correct foundation, building a new religious perspective based on the best aspects of the past with a focus on Nature, the Sacred Year, the Ancestors and the gods and goddesses of Ancient Europe.

4. ADF Druidism:

“DRUID” was the word in ancient Gaul and Britain for the magician-priests who served the religious needs of their tribe. In ADF, we use the term Druid in a similar but modern sense. We are not ancient pagans, we are Neopagans interested in seving our Neopagan communities by providing quality public Neopagan religious celebrations and services. Although our organization has many functions promoting personal spiritual development and private religious work, ADF’s primary mission has always been to offer an accessible and non-exclusive form of pagan worship and celebration to our local communities.

Admittedly we are not practicing “Ancient Celtic Druidism” – nor are we trying to reconstruct it since no one knows what that was like but we are fulfilling a clearly “druidic” role within contemporary Neopaganism, therefore ours is by definition a NEOPAGAN Druidism.

-- Oh well, yes, there is one thing more...

5. The Druids as Depicted in Comic Books and Horror Movies:

"The bloody sacrifices of the Druids" -- embarrassingly hypocritical Christian propaganda against the ancient Druids based on the Druids allegedly officiating at the executions of criminals and prisoners of war (read Julius Caesar's accounts). Such characterizations would be no less appropriate if applied to the clergy of the Roman Catholic Church instead of the Druids.