1975—Original D&D, but not playable
The original Greyhawk supplement included druids, but they weren’t playable characters. They had a shape-changing ability.
1976—Original D&D, as cleric subclass
To the major class of clerics there is also a new subclass, the DRUID. These are similar to the monster of the same name as described in GREYHAWK (Supplement I)
(Eldritch Wizardry, pg. 1, emphasis original)
(Notably, Eldritch Wizardry goes on immediately thereafter to note that “monster” here means what we would today call a “non-player character.”)
The 1976 druid has many familiar spells—produce flame, heat metal, call lightning, control winds, and so on. And they do gain the ability to shape change “Upon reaching the 5th Circle.” The description emphasizes a connection to nature and a requirement to revere and protect nature, and they must be true neutral. The prohibition on wearing metal armor was found even this early, as did the association between druids and “curved blade” weapons, with sickles and scimitars both getting mentioned.
1978—Advanced D&D 1e, again as cleric subclass
Similar to the Eldritch Wizardry druid, the druid found in the AD&D 1e Player’s Handbook was a cleric subclass. Still true neutral, still prohibited from metal armor, etc. Their weapon choices were particularly limited, though again the scimitar makes an appearance. In a lot of ways, the AD&D druid seemed to be a kind of halfway point between magic-user and cleric, being able to wear armor but not the best stuff, getting both attack and healing spells but not at the same levels, etc. etc.
1984—BECMI D&D, as a cleric option
The “C” of BECMI, the Companion Set, introduced rules for levels 15th through 25th as well as several new character classes, including the druid. Clerics of 9th level could become druids, and by adhering to a code of conduct got some special “druidic” spells and powers.
1989—AD&D 2e, “priest” subclass
In its second edition, AD&D made both clerics and druids subclasses of the “priest” class, and thus the two are quite a bit more similar to one another than they were in OD&D or AD&D 1e.
2000—D&D 3e, independent class
The 3e druid was very similar to the druid of 3.5e and 5e, but was notably hampered in a number of ways, and was stated to be among the chief reasons for eventually pushing for the “3.5 revised edition.”
2003—D&D 3.5e, independent class
The D&D 3.5e druid is largely an extension of the D&D druid as it had long been, really right back to 1976. Many of the same spells were there, as was the emphasis on shape changing with the wild shape ability. Like AD&D 2e, the cleric and druid shared similar spell progressions, and were the two original divine “full-casters.”
A big new development was the animal companion—in 3e, druids could get an animal companion by casting the correct spells and performing the correct rituals, but it was a complicated, cumbersome process, and not really called out as an important druid thing, just one more spell you might cast. Making the animal companion a core class feature for the druid allowed Wizards to tighten up the language and make it easier to use.
In D&D 3.5e, druids are one of the “Big 5,” the most powerful classes in the game. This came primarily from its spellcasting—“full” spellcasting, as also found on the cleric and wizard, is incredibly dominant in this edition. But wild shape and the animal companion were also extremely powerful features—even if it was a distant third in terms of a druid’s abilities, the animal companion could often out-fight entire characters of other classes.
2009—D&D 4e, independent class
Despite being a core class in AD&D 1e and 2e, as well as D&D 3e and 3.5e, D&D 4e did not get a druid class until Player’s Handbook 2 was released in 2009. In 4e parlance, the druid was a “primal” “controller” class. Primal referred to its power source, being all about natural magics and the like as usual, and controller referred to its party role, being battlefield control through the use of area effects and minions. Wild shape returned, and between that and the general trend for Primal classes to be a bit tougher than others, the druid was somewhat more comfortable in the thick of things than a typical controller like the wizard.
While minions were a big part of how the 4e druid controlled the battlefield, it did not have a constant animal companion as the 3.5e druid had.
However, Heroes of the Forgotten Kingdoms, an “Essentials” product, offered the “Sentinel” subclass for the druid. Almost an entirely different class, the sentinel is a leader rather than a controller, and does get an animal companion—but no wild shape.
2014—D&D 5e, independent class
A new edition, a new Player’s Handbook, and a new druid, since it was once again a core class. The 5e druid is extremely similar to the 3.5e druid... taken piecemeal. The introduction of Druidic Circles and the placement of druid class features into one circle or another meant that, even though all the features are there, no one druid can have all of them. And actually, not all the features are there, because the 5e druid has no animal companion—that feature is now the exclusive province of the beastmaster ranger.
Otherwise, though, the druid is still full spellcaster, still nature themed with many of the same spells, and still has wild shape. The Circle of the Moon druid, in particular, is again among the strongest classes in the game.