The Druid class in AD&D 1st edition is one that represents a combination of nature based clerics and the Celtic druids of old. Without worshipping any specific pantheon, the Druid is a worshipper of the sun, moon, and all things in nature. To quote the Players Handbook:

“Druids can be visualized as medieval cousins of what the ancient Celtic sect of Druids would have become had it survived the Roman conquest.”

The Druid is a sub-class of the Cleric. For some Players, the Druid is another novelty class or chosen as a flight of fancy. 

On paper, the Druid can be anything from the protector of nature to a borderline eco-terrorist. Where the Cleric follows a single deity or a pantheon and attempts to promote a belief or way of life, the Druid wants nature to run its course with as little interference as possible.

The Druid also looks for signs and works on behalf of the cosmos, attempting to bring the universe into some much needed balance.

Character Ability Scores

The Druid class requires the following minimum ability scores: Wisdom 12 and Charisma 15. 

On the face of it, it may seem odd (not perhaps for Wisdom minimum as it is a Cleric based class), but the Charisma score. It is worth keeping in mind these are the primary abilities connected to a Druid.

It’s also worth noting, if both of these ability scores are 16 or higher the Druid will receive 10% bonus to earned experience.

Back to the question of abilities, scores, and the basis of the class (in the mind's eye of the creators); we look to the original Celtic Druids this Druid class is built upon. 

Julius Caesar (yes, THE Julius Caesar), wrote with great respect about the druids of old; they were astronomers, law bringers, and councillors to the tribes. According to his diaries, their counsel was sought before every major event, to bless (crops and the like), divine an outcome, and most importantly, Julius Caesar witnessed how a word from the Druid could stop two forces mid-battle.

In AD&D terms, in a party of unruly adventurers, the Druid will be the calm.

The high Charisma and nature loving aspect of the class means that every farming town or community will want to hear what the Druid has to say, or beg for advice.

Mistletoe

The ‘holy symbol’ and primary spell component of the Druid is mistletoe. Many of the spells used by the Druid will have a mistletoe component unless stated otherwise. The Mistletoe has to be harvested in a particular way: harvested in the Midsummer’s eve with a gold or silver sickle and caught in a bowl before it touches the ground.

Of course it is up to the DM to decide whether any or all of the conditions are required (such as differences between greater and lesser mistletoe).

Experience

The Druid class gains levels faster than most other classes, the 10,000 X.P. challenge places them at 4th Level and at 100,000 X.P. the Druid is 9th Level compared to the Cleric’s 7th Level of experience (Magic-users, Illusionists and Thieves are at 8th).

As the Druid has a maximum level limit of 14th Level, the X.P. bottle narrows considerably during the middling levels and then widens as the maximum level is reached.

From 12th Level the Druid needs to compete against the standing Druids in order to progress. Assuming the Druid survives the contest, failing the contest reduces the Druid to the level below, and though these contests do not always have to be ‘to the death’, when summoning Insect Plagues or Conjuring Elementals, sometimes these things do happen.

Only two races can be played as a Druid: Human and Half-elf.

Aside from the special abilities of the Half-elf, there is no difference to the races pertaining to the Druid.

The Half-elf is not limited by race to a level.

The Druid cannot be multi-classed.