The key difference between early RPGs and the miniature wargames they diverged from is a simple one: the idea that each player controls not an army, squad, or other group, but a single character. Of course, this means that instead of generating or selecting an army, players must generate or select characters to play. Characters are defined largely in terms of their attributes, class, race, and level, each of which we will cover in turn.
D&D, both the original game and all subsequent versions, has six 'attributes' that express the raw physical and mental qualities of characters. These are, in the order the original game gives them: Strength, Intelligence, Wisdom, Constitution, Dexterity, and Charisma. Attributes and class are inter-related, with three of the attributes being the prime requisite of a class.
The attributes are assigned by rolling 3d6 for each one, in order. OD&D gives little explanation of its attributes, and makes very little mechanical use of them. Let's go through them and what they're used for in the OD&D rules quickly:
Strength is the prime requisite for fighting-men: high Strength gives them experience bonuses, and low gives penalties. It is also of assistance to clerics, who get to add 1/3 their Strength to their Wisdom when determining their experience bonus or penalty. Its description says it "will also aid in opening traps and so on." However, no actual mechanics are either given or suggested for this. In fact, beyond giving fighting-men an experience bonus, Strength has no mechanical uses whatsoever described in OD&D - it doesn't help with attacks, doesn't increase damage, and doesn't even influence the maximum weight a character can carry. The first OD&D supplement, Greyhawk, would rectify all this, but it seems a curious omission.
Intelligence is the prime requisite of magic-users. Fighting-men and clerics both get to add 1/2 their Intelligence to their own prime requisites to determine experience adjustments. It also allows additional languages: characters speak two languages to start with (the 'common tongue' and their alignment language), but can add another for each point of Intelligence over 10 they have. This, however, was all the mechanics that were associated with it - it did not at this point influence what spells a magic-user could learn. This, too, would be added in Greyhawk.
Wisdom is the prime requisite for clerics, but assists both fighting-men (adding 1/3 its value to their Strength for experience purposes) and magic-users (who get to add 1/2 its value). Here too, there were no other actual mechanics associated.
Constitution gave a bonus or penalty to hit points (+1 or -1 per hit die), and gave a percentage chance of "surviving adversity".
Dexterity could give a +1 or -1 penalty to missile attacks.
Charisma actually had more mechanics associated with it than any other attribute: it determined the maximum number of hirelings a character could have, and modified the "loyalty base" of NPCs relative to the PCs they were hired by or were henchmen of. This in turn modified morale rolls, and Charisma also modified reaction checks.
Unlike later versions of D&D, the original game gives no guidance on how to use the attribute numbers to help determine the success or failure of actions. This led to individual gamemasters each creating their own methods to figure such things... and, following the example of the mechanics given in D&D, often completely ignoring the attributes in those mechanics.
The simple expedient of rolling a d20 and trying to roll under the attribute seems to have not been much used. Indeed, the first issue of The Dragon (June 1976) has an article on "How to Use Non-Prime-Requisite Character Attributes", which presented a system which by modern standards is almost comical in its complexity: the GM is to roll 1d100 + attribute, use that to consult a table, roll a d4, d6, d8, d10, or d12 based on that, and multiply the attribute by the result of that die... then use that result as the percent chance of success.
As we've seen, the three classes of OD&D are fighting-man, magic-user, and cleric. Although the thief or rogue would come to be considered one of the core classes of the game, it was officially added to the game in Greyhawk.
The fighting-man and magic-user hold obvious and well-known places in fantasy literature. The cleric, however, doesn't really seem to correspond to anything in the most popular and well-known fantasy literature - there's no cleric-like characters in The Lord of the Rings, nor in Leiber's Fafhrd and Grey Mouser stories, nor in Howard's Conan stories.
According to players in Dave Arneson's early games, the cleric was originally the 'priest', and the main inspiration for it was Van Helsing in the Hammer films - one player wished to play a vampire hunter, and his character was given abilities to wield holy powers, and to do some healing.
Gygax changed this to the cleric, and here we see his apparent love of medieval lore. The term 'cleric' itself originates in Ecclesiastical Greek klerikos "of or belonging to the clergy". It was Latinized as clericus, and as such, was used throughout the medieval period to refer to clergy. He also drew on the medieval Knights Templar and Knights Hospitaller, plus Victorian stories that indicated that medieval clergy who wished to participate in battle would use clubs or maces as weapons, to get around a prohibition against priests shedding blood. Mixed into this, we get clerical magic drawn from medieval saint stories and miracles described in the Bible - so D&D clerics produced food and water, healed the injured, cured disease, raised the dead, turned sticks to snakes, and so forth.
Thus, we get the OD&D cleric: a holy warrior with saint-like abilities, but limited to using only certain weapons. Their magic was treated essentially the same as that of magic-users, but with a different list of spells available to them, and with clerics receiving the bonus that they did not have to find and learn new spells - upon gaining a new spell level, they immediately gained access to the clerical spells of that level.
Magic and Magic-Users
Chainmail already had a magic system, for its 'wizards', consisting of ten spells. These were: phantasmal forces, darkness, wizard light, detection, concealment, conjure elemental, move terrain, protection from evil, levitate, slowness, haste, polymorph, confusion, hallucinatory terrain, cloudkill, and anti-magic shell. Each spell had a 'complexity' from 1 to 6, and less powerful wizards could not cast those of higher complexity.
In addition to these, wizards could throw either fire balls or lightning bolts (which one had to be selected before play). Wizards were also impervious to normal missile fire, and could counter the magic of other wizards (which became the D&D spells protection from normal missiles and dispel magic). They came in five different degrees of power, from a Seer who could only use one spell, to a full Wizard who could use six or seven spells. However, exactly how this worked is very unclear in Chainmail - there's nothing which says whether a Seer gets one spell that can be used over and over, or whether they can simply only cast one spell from the above spell list in the course of a game, but can freely choose which one at the time of casting. (The fire ball and lightning bolt aren't listed as being spells per se in Chainmail, and thus, can be used repeatedly.)
All this made Chainmail wizards terrifyingly powerful on the battlefield... but that was all right, because Chainmail balanced fantasy forces by means of a point system. A full wizard was worth 100 points, the same as a dragon, and more than any other type. Heroes were only worth 20 points, making a wizard as valuable as five of them!
D&D, however, had to balance things between individuals... and so, while many of the elements of Chainmail's wizards were retained, others were dropped. A Chainmail wizard was quite powerful in melee, being counted as two Armored Foot or two Medium Horse (if mounted), and capable of using any sort of magical weapon, including magic swords. On the Fantasy Combat Table, they are hard to kill, with Giants and Super-Heroes (defined to be exceptionally powerful heroes, such as Conan) being the only types who are more likely to kill a wizard than to be killed by one.
All this went by the wayside for the D&D magic-user. They were vastly demoted in combat ability, being the weakest of the three classes in melee, having the fewest hit points, and being unable to use most weapons (whether magical or not) or any armor. The spell 'complexity' of Chainmail became spell levels, and were used to control how powerful of a spell a magic-user could cast, dependent on their own level (although not in a simple one-to-one relation).
Most importantly, though, the magic-user was limited in the number of times a day they could use each spell. For this, Gygax borrowed a concept from Jack Vance's Dying Earth novels, wherein spells were extremely complex and alien formulae, which a magician had to devote considerable time to memorizing... and which, when used, would sort of burn themselves out of the magician's memory, so they would be forgotten and have to be laboriously re-memorized.
However, this fact was only mentioned once in the rulebooks (in the first paragraph on page 19 of Men & Magic), and seems to have been very easy to miss. It was either missed or ignored enough that Gygax regularly felt the need to call attention to it, doing so in an FAQ about D&D rules presented in the second issue of The Strategic Review (Summer 1975), and again less than a year later in the article "The D&D Magic System" in the April 1976 issue (TSR had switched from quarterly to bi-monthly along the way). Thus, in campaigns where the Vancian system was not in use, the magic-user frequently outstripped other characters in power.
It should be noted too that in original D&D, the starting spell selection for magic-users had no direct combat spells. Such spells as magic missile, burning hands, and even shield did not exist yet. The most useful spells that starting magic-users had for combat were protection from evil, sleep, and charm person. Phantasmal forces and invisibility were in the list of second level spells, but magic-users didn't gain spells that could cause hit point damage until the could cast the third level spells fireball and lightning bolt... when they themselves were fifth level. Lower level magic-users wouldn't gain officially sanctioned damaging spells until the release of Greyhawk.
... and that's it for this time. We haven't really touched on the fighting-man class yet, but it makes more sense to save that for Part Four, where we'll dive into OD&D Combat!
Gygax, Gary and Arneson, David, Dungeons & Dragons. 1st edition, Tactical Studies Rules, 1974.
Gygax, Gary and Perren, Jeff, Chainmail, 3rd edition, Tactical Studies Rules, 1975.