What is D&D?
Dungeons & Dragons (D&D) is a tabletop role-playing game that emerged in 1974. In D&D, a group of players form an adventuring party to explore fantasy worlds and engage in quests presented by the game's referee and storyteller: the Dungeon Master (DM). There is no immediate way to win or lose the game, save a type of completion of the story through completing a series of given quests which may take weeks or even years.
Essentially, any conceivable story could be turned into a D&D campaign, at the DM's discretion and ability. The DM also creates the landscapes, acts out different characters, and even plays against the party as enemies.
The players make decisions based on what the DM is describing and showing the group, working through problems that don't always have an obvious answer. Amazingly, any decision a player could ever want to make is possible through the game's interface and rules.
How does it work?
A set of universal rules govern what players and DMs can and can't do. Both players and monsters have statistics that affect how well they can execute decisions. For example, someone with +3 dexterity will usually be successful in performing tasks that require grace and agility.
Any decision made by a player, or a DM, can be categorized into one of the 20 "skills"; from Acrobatics to Survival, players will roll a twenty-sided dice (D20), add their statistic score to the roll, and measure up against the DM or monster's difficulty level to see if the skill is successful.
For example, do you want to try to steal the key out of the guard's pocket? Roll a "Sleight of Hand" check and add your modifier (+5) to see if you can. Now, the guard is on alert so you must roll an 18 or higher to be successful. Want to ask for the key? Roll Persuasion. Want to threaten the guard to give you the key? Roll Intimidation.
In this way, any decision you would ever want to make is possible through this system.
The Interface
The mechanics of D&D are compiled in several volumes of rulebooks, guides, and manuals. The most important books for first-timers are the 'how to make your character guide' and the 'collection of monster stats manual'. There are over 50 D&D books covering various topics, so many people frequently print and recategorize their own copies.
Making Your Character
Yes, the rules are so expansive that it requires a full textbook of information, just to make a single character, before you even start playing. Decisions such as class, race, and background all affect the statistics of your character. For example, a rogue class will have a high ability to sneak and be stealthy; a halfling rogue will have an even higher ability because they are half the size of a normal person and easily missed. So, making your character has exponential cause-and-effect possibilities.
A classic D&D character sheet will include stats, skills, weapons, inventory, abilities, personal information, and more. A usual D&D sheet is filled with pencil scribbles and eraser marks because as a character completes quests, they also gain experience points and level up!
Spells and Abilities
This is probably the most complex area of D&D as each time a player uses a spell or ability, a complex understanding is required because all moments are contextual. For example, I want to attack the person in the dark woods with Fireball. Since I cannot see them clearly, I attack with disadvantage; however, I will use my bonus action Tides of Chaos to attack with advantage, which means I can attack normally.
There are pages, and pages, and pages of text dedicated to knowing how to use spells and abilities properly in each of these contexts.
Finally Playing
Most people play at a table with some type of visual interface between players. As positioning and context are important, it helps players understand what is happening. Some DMs will construct 3-dimensional models to play within, many use dry-erase sheets which are much less time-consuming, and some don't use visuals and leave it all up to the imagination!
Clearly, D&D is a complex game. It takes years of learning to master. However, it is changing, maybe for the better!