I am going to talk the original edition of Dungeons & Dragons, the first role-playing game ever.
The game was a "fantasy system with medieval European flavor". The players could choose 4 races (humans, dwarf, elf, hobbit [this was changed to "halfing" due to legal reasons]), 3 character classes (fighters, cleric, magic user), 3 alignments (chaotic, lawful, neutral) and a handful of monsters. The game also other concepts that would become standard like magic items, treasure, armor class, subterranean dungeons full of traps and abilities (strength, intelligence, dexterity, etc.). However, the game was difficult (if not impossible) if you never played Chainmail wargame, Outside Survival board game or any table top war game. However, Greyhawk supplement supplied new combat rules that made it easier for new players. This led to an entire industry of role-playing and both offical and unoffical supplements.
My sources were the following Wikipedia pages: "Dungeons & Dragons (1974)"; "Editions of Dungeons & Dragons", section "Edition and version history", sub-section "Dungeons & Dragons"; and "Greyhawk (supplement)".