On 26th January 1974 at 1.40pm EST (7.40pm GMT), E Gary Gygax invited some friends around his house to play a new game. Based on his Chainmail minatures rules, the game used the Man to Man variant of those rules to run adventures in fantasy dungeons, based on the works of Tolkien, Vance, Leiber etc (although Gygax himself disliked the works of JRR Tolkien, he mined it for ideas, especially Elves, Dwarves, Halflings/Hobbit and Balors/Balrogs, to the extent he was threatened with legal action by the Tolkien estate). The infamous Appendix N of the original Dungeon Master’s Guide gives some idea of the influences on Gygax.
This game became Dungeons and Dragons (named, according to legend, by Gygax’s daughter), which spawned the Role-playing industry. Without D&D you would not have computer Role-Playing Games, at least not in the format they took, with levels, hit points etc (World of Warcraft and most other MMORPGs pay a hugh debt to D&D). Although many RPGs moved away from the conventions of D&D (dispensing with levels, hit points, armour class etc), they are still recognisably the bastard children of E Gary Gygax’s original game.
With D&D itself having gone through four revisions (not counting BECMI in the 80s), and a new version due out this Summer, all I can say is Happy Birthday Dungeons and Dragons!