General Discussion

D&D and all other role playing games are basically interactive fiction run/guided by a human. Think computer rpg but instead of computer, a human is running the game. If you plan to be a player, it's pretty easy and only light knowledge of the rules are needed. If you plan to run the game as a Dungeon Master (DM), then a more in depth understanding is needed, as you are guiding but not completely controlling a story. The tomes are thick but are mostly reference guides and situational resolutions. Those resolutions are most commonly combat but other outcomes are also decided, mostly with dice as each character has a chance to interact with the situation in a positive to the character way. That is a successful sword blow, a chance to jump out of the way of a falling object, a chance to influence a interaction with a non player character (NPC). These chances vary per character and are influenced by skills, levels, character attributes, and items. You don't need to memorize the rules more than understand where to find stuff when you need it. Combat tables, monster descriptions and treasure being the most common. To a good DM, the specific rule set is less important than the consistency of using a single set, so the players understand how to interact with the story and feel it is fair.